Move documentation to a book format

[ehuss]

This moves the documentation in the README to a dedicated book with mdbook. This includes some new content and edits with the intent of better organizing the content and hopefully make it easier for new users. The changes are split in separate commits so you can see the content changes (the first commit should not have any content changes).

A preview can be seen at https://ehuss.github.io/rustup/. Or, you can check out this PR and run mdbook serve in the doc directory.

CI is configured to deploy to GitHub pages on each push to master. This should appear on https://rust-lang.github.io/rustup/. BEFORE MERGING, the github-pages needs to be enabled on the repository. Settings → scroll down to "GitHub Pages" → set source to the "gh-pages" branch → click "Save".

It should be relatively easy to use a custom domain if we want (like rustup.rust-lang.org?), but that would require infra to help.

In the future, the CI can probably be changed to only publish on stable (or possibly some manual mechanism). In the short term, I think publishing from master will be better since there might be changes that will be needed before the next release (and performing the initial publish).

Closes #2396

[kinnison]

I really like this approach and I think the CI looks good. Sorry it took so long for me to come to this. You have a conflict on README because of changes since you made your branch. I wonder if @rbtcollins has any reason to say this wouldn't be a good path to improving docs? If not, then a rebase and incorporation of whatever change is conflicting into your split docs will be needed before we can merge. 👍 on the approach from me.

[kinnison]

@ehuss if you're able to solve the conflict (incl. updating anything into your split content) then we can merge 🎉

[ehuss]

I have rebased.