docs: (readme) link to datadir doc from cfp; (index) add guide (#150)

- `Guide` page is not a child of `services-flake` in
<https://community.flake.parts> page:

<img width="288" alt="Screenshot 2024-03-13 at 3 54 20 PM"
src="https://github.com/juspay/services-flake/assets/23645788/58f31702-ac4b-4338-83eb-e7a69c1ea705">


- Link to a more detailed datadir documentation in README

README.md

@@ -43,9 +43,9 @@ Want to compare with other tools? [Let us know](https://github.com/juspay/servic
 [^5]: `service-flake` exposes [`process-compose-flake`](https://github.com/Platonic-Systems/process-compose-flake) modules for each service, which can be reused as long as your project is using `flake-parts`. With devenv, decoupling is not possible (nor [prioritized](https://github.com/cachix/devenv/issues/75#issuecomment-1324914551) in future) unless you buy into the whole devenv ecosystem.
 
 
-## A note on process working directory
+## Service data directory
 
-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/). The example uses the latter.
+See <https://community.flake.parts/services-flake/guide/datadir>
 
 ## Discussions
 

doc/index.md

@@ -17,6 +17,7 @@ See:
 - [[start]]#
 - [[services]]#
 - [[contributing]]#
+- [[guide]]#
 
 ## Demo