Add autodoc website

This adds a `zig build docs` step that builds the documentation website
and writes it to zig-out/docs.

It further includes a GitHub Workflow that publishes this website
to GitHub Pages. The GitHub Workflow is divided into two jobs:

- build: builds the documentation and uploads it
- publish: downloads the documentation and publishes it

These are separate jobs to minimize permissions available
to the build job.

This workflow runs on two events:

- after every push to master
- `workflow_dispatch`: this allows manually running the workflow
  from its *Actions* page if something went wrong

---

**Important pre-merge steps:**

If this PR is accepted, the following steps should be taken
before merging the PR:

1. Go to **Settings** for the repository
2. Select **Pages** on the left under *Code and automation*
3. Under *Build and deployment* set **Source** to **GitHub Actions**
4. Merge the PR.

If the steps are missed, the PR will merge just fine,
but the docs job will fail immediately on merge.
This can be remedied by following steps 1-3 above,
and either adding a new commit on master,
or manually firing the workflow from the Actions > API Reference page.

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