Update the Git integration doc.

[Niloth-p]

Moved from #32780, following the migration of the integration docs of the API integrations.

Updates since last review

1. Warning note: "This integration is meant to be executed on your Git server." -> "This integration adds a post-receive hook to your Git repository on your Git server."
2. Rewritten instruction: "Symlink the post-receive hook of your Git repository by running:" -> "If your Git server and your Zulip server are on the same machine, symlink the post-receive hook of your Git repository in your Git server by running: ...command... Otherwise, copy the post-receive hook to your Git repository's /hooks directory."
3. Converted the tip "The post-receive hook is triggered on every push to the repository." to a line of text inside the instruction.
4. Opened #34494 to update the installation macro.

Screenshot

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[Niloth-p]

@laurynmm Could you please review this?

[laurynmm]

@Niloth-p - Thanks for moving these updates from the other repository! I had a few comments, but overall looks good. Let me know if you have any questions about my feedback!

[laurynmm]

With the expanded instructions for step 3, it seems like we could just drop this note/warning.

[laurynmm]

Noticed in the screenshot of the rendered documentation that this is the API URL and not the URL for the Zulip organization and I just wanted to be sure that's what we want shown here.

[laurynmm]

Does it make sense to show the default since there isn't a screenshot?