docs(guide): dataDir init; unix-domain socket length limitation (#144)

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Co-authored-by: Sridhar Ratnakumar <3998+srid@users.noreply.github.com>

doc/guide.md

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+
+# Guide
+
+- [[datadir]]#

doc/guide/datadir.md

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+# Data directory
+
+`dataDir` is a an option present in all the services, allowing users to specify the directory where a given service will store its data files. Essentially, it is to persist the state of the service even when you exit the `process-compose` window.
+
+The `dataDir` of these services tend to take *relative* paths, which are usually relative to the project root. As such, when you run these services using `nix run`, their data files are created relative to whichever directory you are in. If you want these data files to always reside relative to the project directory, instead of using `nix run` consider wrapping the process-compose packages in script, via either [mission-control](https://community.flake.parts/mission-control) module or a [justfile](https://just.systems/). `services-flake` uses the latter.
+
+{#default-structure}
+## Default data directory structure
+
+Let's say your project defines the following services:
+
+```nix
+{
+    # Inside `perSystem.process-compose.<name>`
+    services.postgres.pg.enable = true;
+    services.redis.rd.enable = true;
+}
+```
+
+The data directory structure will look like this:
+
+```sh
+|-- data
+|   |-- pg
+|   |-- rd
+```
+
+## Gotchas
+
+{#socket-path}
+### Unix-domain socket path is too long
+
+unix socket length is limited to [about 100 chars](https://linux.die.net/man/7/unix). If your data directory is nested too deep, you will have to set `dataDir` option of the service to a shorter path as a workaround.