docs(readme): Simplify README and link to detailed docs

[github-actions]

Closes #443

Applied changes based on Issue #443.

I’ve slimmed down the README into a lean “landing page” and moved all of the deep‑dive material into a new docs/ folder. Here’s what changed:

• README.md:
  • Kept only the title, badge ribbon, a one‑sentence elevator pitch.
  • Added a concise Documentation section linking to the individual guides in docs/.
  • Added a minimal Quickstart workflow snippet.
• docs/installation.md: “Installation” guide with project settings & workflow configuration.【F:docs/installation.md†Entire File】
• docs/usage.md: All of the /codex example‑usage scenarios (Issues, PRs, creating issues).【F:docs/usage.md†Entire File】
• docs/configuration.md: Tables of basic inputs, Codex inputs, advanced Codex inputs.【F:docs/configuration.md†Entire File】
• docs/security.md: Permission checks & secret‑masking security notes.【F:docs/security.md†Entire File】
• docs/developing.md: Developer‑focused “running tests”, formatting, linting, branch‑naming guidelines.【F:docs/developing.md†Entire File】

These moves keep the README glance‑able while housing all the detailed docs in their own files. Let me know if you’d like any tweaks!