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

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.

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.

By doing this, make the GitHub directory listing for the documentation
directory more useful and less cluttered.

Closes Homebrew/homebrew#48219.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Remove duplication, link to the API documentation more often,
tweak wording, add `@UniqMartin` as a maintainer, note `@jacknagel`'s
and `@adamv`'s significant past contributions to Homebrew, delete some
outdated or unneeded documentation, add some missing `Formula` API.
entries and simplify/improve `CONTRIBUTING.md`.

Closes Homebrew/homebrew#46179.

Closes Homebrew/homebrew#46618.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes Homebrew/homebrew#35529.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Closes Homebrew/homebrew#33211.