docs: write README with features and workflow overview

README.md

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+# improved-octo-fortnight
+
+A reference implementation of a documentation workflow using [zensical](https://zensical.org/) and GitHub Pages.
+
+- [latest](https://taisakuma.github.io/improved-octo-fortnight/latest/)
+- [dev](https://taisakuma.github.io/improved-octo-fortnight/dev/)
+
+## Features
+
+- **Versioned docs** -- Release docs at `/<version>/`, dev docs at `/dev/`, and a `/latest/` alias
+- **PR previews** -- Every PR gets a preview deployment at `/pr/<number>/` with an automatic comment linking to it
+- **PR cleanup** -- Preview deployments are removed when PRs are closed
+- **Version index** -- An auto-generated root `index.html` lists all available versions
+- **Deploy verification** -- The preview URL is polled before posting the PR comment; a note is added if the site isn't ready yet
+- **Custom domain support** -- Works with both default GitHub Pages URLs and custom domains
+- **API reference** -- Auto-generated from Python docstrings using [mkdocstrings](https://mkdocstrings.github.io/). Add `:::` directives in Markdown to render any module (see `docs/api.md` for an example)
+
+## How it works
+
+Documentation is built with [zensical](https://zensical.org/) and deployed by pushing subdirectories to a `gh-pages` branch. Four workflows handle the different deployment scenarios:
+
+| Workflow              | Trigger                          | Deploys to                                |
+| --------------------- | -------------------------------- | ----------------------------------------- |
+| `docs.yml`            | Push to `main`                   | `/dev/`                                   |
+| `docs-release.yml`    | Release (via changelog workflow) | `/<version>/` + `/latest/` + `index.html` |
+| `docs-pr-preview.yml` | PR opened/updated                | `/pr/<number>/`                           |
+| `docs-pr-cleanup.yml` | PR closed                        | removes `/pr/<number>/`                   |
+
+The `gh-pages` branch has the following structure:
+
+```
+gh-pages
+├── .nojekyll
+├── index.html          # auto-generated version index
+├── latest/             # copy of the latest release
+├── dev/                # built from main
+├── 0.2.1/              # release versions
+├── 0.2.0/
+└── pr/
+    ├── 14/             # PR preview
+    └── 13/
+```
+
+Shared logic is extracted into two composite actions:
+
+- **`.github/actions/build-docs`** -- Overrides `site_url` for the target subdirectory, builds the site, and stages the output
+- **`.github/actions/deploy-to-gh-pages`** -- Checks out (or creates) the `gh-pages` branch and pushes the built site to a target subdirectory
+
+A Python script (`.github/scripts/override_site_url.py`) uses [tomlkit](https://github.com/sdispater/tomlkit) to modify `site_url` in `zensical.toml` so that navigation and search work correctly in each subdirectory.
+
+## Setup
+
+1. Install [uv](https://docs.astral.sh/uv/)
+2. Build docs locally:
+   ```
+   uv run --group docs zensical serve
+   ```
+3. Set GitHub Pages source to **Deploy from a branch** > `gh-pages` (root `/`)