docs(custom-service): Document the use of a custom service definition (#243)

Resolves https://github.com/juspay/services-flake/issues/242

Preview:
```sh
nix run "github:juspay/services-flake/custom-service?dir=doc"
```

doc/custom-service.md

@@ -0,0 +1,166 @@
+# Custom service
+
+When using `services-flake` you are not just limited to the [[services|builtin services]]. You can also define your own service.
+
+By default, `services-flake` supports multiple instances for each service, allowing you to run several instances of the same service simultaneously. However, you also have the option to create custom single-instance services. In the following sections, we’ll explore how to define custom services of both types.
+
+{#single-instance}
+## Single instance service
+
+We will create a `hello` service that will return a greeting message:
+
+```nix
+{ config, lib, pkgs, ... }:
+{
+  options = {
+    services.hello = {
+      enable = lib.mkEnableOption "Enable hello service";
+      package = lib.mkPackageOption pkgs "hello" { };
+      message = lib.mkOption {
+        type = lib.types.str;
+        default = "Hello, world!";
+        description = "The message to be displayed";
+      };
+    };
+  };
+  config =
+    let
+      cfg = config.services.hello;
+    in
+    lib.mkIf cfg.enable {
+      settings.processes.hello = {
+        command = "${lib.getExe cfg.package} --greeting='${cfg.message}'";
+      };
+    };
+}
+```
+
+Let's call this file `hello.nix`.
+
+Now, we can import this service in our flake. In this example, we will configure an existing service, [[ollama]], and our custom service from above:
+
+```nix
+{
+  description = "A demo of importing a single instance custom service";
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    systems.url = "github:nix-systems/default";
+    process-compose-flake.url = "github:Platonic-Systems/process-compose-flake";
+    services-flake.url = "github:juspay/services-flake";
+  };
+  outputs = inputs:
+    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = import inputs.systems;
+      imports = [
+        inputs.process-compose-flake.flakeModule
+      ];
+      perSystem = { self', pkgs, lib, ... }: {
+        process-compose."default" = { config, ... }: {
+          imports = [
+            inputs.services-flake.processComposeModules.default
+            ./hello.nix
+          ];
+
+          services.ollama."ollama1".enable = true;
+          services.hello.enable = true;
+        };
+      };
+    };
+}
+```
+
+Finally, `nix run`:
+![[single-instance-hello.png]]
+
+{#multi-instance}
+## Multi-instance service
+
+For this purpose, `services-flake` exports a [multiService](https://github.com/juspay/services-flake/blob/647bff2c44b42529461f60a7fe07851ff93fb600/nix/lib.nix#L1-L34) library function. It aims to provide an interface wherein the user just writes the configuration like they would for a single instance service, and the library takes care of creating multiple instances of the service.
+
+Let's write the same `hello` service as above, in `hello.nix`, but this time as a multi-instance service:
+
+```nix
+{ config, lib, name, pkgs, ... }:
+{
+  options = {
+    enable = lib.mkEnableOption "Enable ${name} service";
+    package = lib.mkPackageOption pkgs "hello" { };
+    message = lib.mkOption {
+      type = lib.types.str;
+      default = "Hello, world!";
+      description = "The message to be displayed";
+    };
+    outputs.settings = lib.mkOption {
+      type = lib.types.deferredModule;
+      internal = true;
+      readOnly = true;
+      default = {
+        processes.${name} = {
+          command = "${lib.getExe config.package} --greeting='${config.message}'";
+        };
+      };
+    };
+  };
+}
+```
+
+The primary differences from the single instance service are:
+
+- The module now takes an additional argument `name`, which is the name of the instance of the service.
+- We no longer have to write the `config` block, as it is now handled by the library by importing the `outputs.settings` option.
+- And we don't have to write `options.services."${name}"`, as that is abstracted away by the library.
+
+Now that we have defined the multi-instance service, we can import it in our flake:
+
+```nix
+{
+  description = "A demo of importing a multi-instance custom service";
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    systems.url = "github:nix-systems/default";
+    process-compose-flake.url = "github:Platonic-Systems/process-compose-flake";
+    services-flake.url = "github:juspay/services-flake";
+  };
+  outputs = inputs:
+    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = import inputs.systems;
+      imports = [
+        inputs.process-compose-flake.flakeModule
+      ];
+      perSystem = { self', pkgs, lib, ... }: {
+        process-compose."default" = 
+        let
+          inherit (inputs.services-flake.lib) multiService;
+        in
+        {
+          imports = [
+            inputs.services-flake.processComposeModules.default
+            (multiService ./hello.nix)
+          ];
+
+          services.ollama."ollama1".enable = true;
+          services.hello = {
+            hello1 = {
+              enable = true;
+              message = "Hello, world!";
+            };
+            hello2 = {
+              enable = true;
+              message = "Hello, Nix!";
+            };
+          };
+        };
+      };
+    };
+}
+```
+
+And finally, `nix run`:
+![[multi-instance-hello.png]]
+
+## See also
+
+- [Postgres with replica](https://github.com/nammayatri/nammayatri/blob/main/Backend/nix/services/postgres-with-replica.nix)
+- [Passetto (A custom encryption service)](https://github.com/nammayatri/passetto/blob/nixify/process-compose.nix), is [imported](https://github.com/nammayatri/nammayatri/blob/e8032f1fac3581b9062e2469dfc778d2913d3665/Backend/nix/services/nammayatri.nix#L32) and [configured in the Nammayatri flake](https://github.com/nammayatri/nammayatri/blob/e8032f1fac3581b9062e2469dfc778d2913d3665/Backend/nix/services/nammayatri.nix#L285-L297).

doc/index.md

@@ -20,6 +20,7 @@ See:
 - [[services]]#
 - [[contributing]]#
 - [[guide]]#
+- [[custom-service]]#
 
 ## Demo