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/install.md

@@ -0,0 +1,40 @@
+## Install & build
+
+### System requirements
+
+| Requirement                 | Details                                                         |
+| --------------------------- | --------------------------------------------------------------- |
+| Operating systems           | macOS 12+, Ubuntu 20.04+/Debian 10+, or Windows 11 **via WSL2** |
+| Git (optional, recommended) | 2.23+ for built-in PR helpers                                   |
+| RAM                         | 4-GB minimum (8-GB recommended)                                 |
+
+### DotSlash
+
+The GitHub Release also contains a [DotSlash](https://dotslash-cli.com/) file for the Codex CLI named `codex`. Using a DotSlash file makes it possible to make a lightweight commit to source control to ensure all contributors use the same version of an executable, regardless of what platform they use for development.
+
+### Build from source
+
+```bash
+# Clone the repository and navigate to the root of the Cargo workspace.
+git clone https://github.com/openai/codex.git
+cd codex/codex-rs
+
+# Install the Rust toolchain, if necessary.
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+source "$HOME/.cargo/env"
+rustup component add rustfmt
+rustup component add clippy
+
+# Build Codex.
+cargo build
+
+# Launch the TUI with a sample prompt.
+cargo run --bin codex -- "explain this codebase to me"
+
+# After making changes, ensure the code is clean.
+cargo fmt -- --config imports_granularity=Item
+cargo clippy --tests
+
+# Run the tests.
+cargo test
+```