docs: fix contributor docs links and typos (#18637)

## Summary

This PR fixes several small documentation issues in the contributor and
setup guides:

- fixes broken docs links in the root README
- corrects multiple typos and capitalization issues in contributor docs
- fixes malformed Markdown for the Redis command in local setup
- improves wording in the Docker Compose self-hosting guide

## Changes

- updated README installation links to the current docs routes
- changed `Open-source` to `open-source`
- fixed `specially` -> `especially` in the frontend style guide
- normalized `MacOS` -> `macOS`, `powershell` -> `PowerShell`, and
`Postgresql` -> `PostgreSQL`
- replaced the invalid `localhost:5432` Markdown link with inline code
- fixed the malformed fenced code block for `brew services start redis`
- cleaned up Redis naming/capitalization and a few grammar issues in the
setup docs
- improved the warning and environment-variable wording in the Docker
Compose guide

## Testing

- not run; docs-only changes

---------

Co-authored-by: Charles Bochet <charlesBochet@users.noreply.github.com>

README.md

@@ -25,8 +25,8 @@
 # Installation
 
 See:
-🚀 [Self-hosting](https://docs.twenty.com/developers/self-hosting/docker-compose)
-🖥️ [Local Setup](https://docs.twenty.com/developers/local-setup)
+🚀 [Self-hosting](https://docs.twenty.com/developers/self-host/capabilities/docker-compose)
+🖥️ [Local Setup](https://docs.twenty.com/developers/contribute/capabilities/local-setup)
 
 # Why Twenty
 
@@ -36,7 +36,7 @@ We built Twenty for three reasons:
 
 **A fresh start is required to build a better experience.** We can learn from past mistakes and craft a cohesive experience inspired by new UX patterns from tools like Notion, Airtable or Linear.
 
-**We believe in Open-source and community.** Hundreds of developers are already building Twenty together. Once we have plugin capabilities, a whole ecosystem will grow around it.
+**We believe in open-source and community.** Hundreds of developers are already building Twenty together. Once we have plugin capabilities, a whole ecosystem will grow around it.
 
 <br />