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.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

Closes #344.

Signed-off-by: Tim D. Smith <git@tim-smith.us>

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outdated or unneeded documentation, add some missing `Formula` API.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>

Always use the `<user/repo>` format as used by the `brew tap` command.
Also fixed a link to `homebrew/head-only` and sorted a list in `FAQ.md`
in alphabetical order.

Closes Homebrew/homebrew#45535.

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

Closes Homebrew/homebrew#35529.

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

prepend_path doesn't do anything if path doesn't already exist, so use
prepend_create_path.

Closes Homebrew/homebrew#34793.

Closes Homebrew/homebrew#34228.