doc: fix relative links and add tips (#7319)

This PR is a documentation only one which:
- addresses the #7231 by adding a paragraph in `docs/getting-started.md`
in the tips category to encourage users to load everything needed in
their environment
- corrects link referencing in `docs/platform-sandboxing.md` so that the
page link opens at the right section
- removes the explicit heading IDs like {#my-id} in `docs/advanced.md`
which are not supported by GitHub and are **not** rendered in the UI:

<img width="1198" height="849" alt="Screenshot 2025-11-26 at 16 25 31"
src="https://github.com/user-attachments/assets/308d33c3-81d3-4785-a6c1-e9377e6d3ea6"
/>

This caused the following links in `README.md` to not work in `main` but
to work in this branch (you can test by going to
https://github.com/openai/codex/blob/docs/getting-started-enhancement/README.md)
- the MCP link goes straight to the correct section now:

```markdown
  - [**Advanced**](./docs/advanced.md)
  - [Tracing / verbose logging](./docs/advanced.md#tracing--verbose-logging)
  - [Model Context Protocol (MCP)](./docs/advanced.md#model-context-protocol-mcp)
```

---------

Signed-off-by: lionel-oai <lionel@openai.com>
Signed-off-by: lionelchg <lionel.cheng@hotmail.fr>
Co-authored-by: lionelchg <lionel.cheng@hotmail.fr>

docs/getting-started.md

@@ -113,3 +113,7 @@ Paste images directly into the composer (Ctrl+V / Cmd+V) to attach them to your
 codex -i screenshot.png "Explain this error"
 codex --image img1.png,img2.jpg "Summarize these diagrams"
 ```
+
+#### Environment variables and executables
+
+Make sure your environment is already set up before launching Codex so it does not spend tokens probing what to activate. For example, source your Python virtualenv (or other language runtimes), start any required daemons, and export the env vars you expect to use ahead of time.