Add "docs" build and CI step

[abhinav]

Preview: A preview of the result of this PR is available at https://abhinav.github.io/ansi-term/

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Adds a zig build docs step to build documentation website, and a new GitHub Workflow that publishes the website to GitHub Pages.

This 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.

To use this, you have to take the following steps:

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

Note that this workflow dispatches on two events:

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

The workflow itself is a separate workflow to make it more copy-paste friendly.

Note on stability: One of the points made by @joachimschmidt557 in #13 was that the generated docs must explicitly call out that the documentation is unstable/WIP.

Because this uses the same tooling as Zig std, the generated pages include the following header at the top:

This is a beta autodoc build; expect bugs and missing information.

Resolves #13

[abhinav]

Note that this will not pass until #19 is merged. The build is currently broken with latest Zig nightly.

[joachimschmidt557]

Thanks a lot!