Add documentation that can be published as gh-pages using mdbook

[sven-of-cord]

This commits adds some additional documentation as markdown pages. That includes a SUMMARY.md files that references the other markdown files in a table-of-contents structure. mdbook can generate a very nice book web site from that. This commit also adds a GitHub workflow that generates the book web site and pushes it to the gh-pages branch for publishing with GitHub Pages.

Test Plan: cargo install mdbook mdbook serve Open http://localhost:3000 in the browser to see the book.

I temporarily added the pull_request trigger to the workflow, so one version of this PR exercised the deploy. You can see the result here: https://getcord.github.io/spr/ and https://github.com/getcord/spr/tree/gh-pages

[joneshf]

This is great!

Could https://getcord.github.io/spr/ be added as the website for the repo? That way it's a bit easier to find the link from the repo. There's not good documentation about it, but it should be under the "About" section on the repo page https://github.com/getcord/spr:

This SO answer talks about it a bit: https://stackoverflow.com/a/62767178

[sven-of-cord]

This is great!

Could https://getcord.github.io/spr/ be added as the website for the repo? That way it's a bit easier to find the link from the repo.

Good idea. Done!

I'll also add a link to the README. The documentation is a bit stealth at the moment. It is very actively worked on right now, we should be able to put up something more complete soon.