README / docs refactor (#2724)

This PR cleans up the monolithic README by breaking it into a set
navigable pages under docs/ (install, getting started, configuration,
authentication, sandboxing and approvals, platform details, FAQ, ZDR,
contributing, license). The top‑level README is now more concise and
intuitive, (with corrected screenshots).

It also consolidates overlapping content from codex-rs/README.md into
the top‑level docs and updates links accordingly. The codex-rs README
remains in place for now as a pointer and for continuity.

Finally, added an extensive config reference table at the bottom of
docs/config.md.

---------

Co-authored-by: easong-openai <easong@openai.com>

docs/open-source-fund.md

@@ -0,0 +1,8 @@
+## Codex open source fund
+
+We're excited to launch a **$1 million initiative** supporting open source projects that use Codex CLI and other OpenAI models.
+
+- Grants are awarded up to **$25,000** API credits.
+- Applications are reviewed **on a rolling basis**.
+
+**Interested? [Apply here](https://openai.com/form/codex-open-source-fund/).**