Docs site for `main` branch

#### Feature

A docs site that is up-to-date with the `main` branch.

#### Benefit

Things move quickly, and people may benefit from being able to see the current API before its released.

#### Implementation

Use GitHub Actions and GitHub Pages to build and host an alternate docs site triggered by updates to `main`.

#### Alternatives

We could use other tools for build/host, but GitHub ecosystem is the most convenient and is already being used.