GitHub provides a description and homepage field so let `brew create`
use them where possible. Also, detect GitHub repositories based on
`releases` as well as `archive`s.

Add `--only` and `--except` methods which can be used to selectively
enable or disable audit groups.

- Remove taps that are (or will shortly be) deprecated.
- Remove commands that are only relevant to the boneyard (which will
  shortly be removed).

Add labeled fences to most code blocks, monospace terms where
appropriate, omit "$" when referring to variables

Also works around bug in Jekyll where a URL with a "#" will cause links
in the same paragraph to not have their extensions replaced by using
"#" instead.

This is useful when you're pulling PRs where individual bottles can't be
uploaded/built but you still want to pull the PR as a whole anyway.

The existing `brew.1.html` wasn't particularly pleasant to read and
given everything else in `docs/` is a Markdown file it makes sense to
generate a post-processed Markdown file that can in turn be used by
Jekyll to generate a nicely themed HTML file.

Note some additional ways to help, gotchas and how to get started.

Also changes some heading levels so only one primary
heading appears per page.

While GitHub allows a single space to denote a sublist, Jekyll is not
so lenient. It also seems unable to handle ordered sublists.

Unfortunately there's no easy way to share this code with the brew.sh
site but it's at least able to have the `css/`, `img/` (root) and
`_layouts` directories in sync between the two repositories.

Some small CSS tweaks were needed to make these pages render more
attractively.

Closes #2186.

Also update command specifications to match descriptions.

Fixes to spelling and punctuation for consistency and readability.

Bare URLs are auto-linked on GitHub but not on docs.brew.sh, so I added
angle brackets around them where appropriate. Also converted
Markdown-linked URLs for clarity.