Fix internal links on the manpage

These previously, incorrectly pointed to bot.brew.sh.

Fixes #2703.

docs.brew.sh: use SCSS.

More environment filtering tweaks

Import style changes from:
https://github.com/Homebrew/homebrew.github.io/pull/166

`venv.link_scripts(bin) { venv.pip_install buildpath }` is now `venv.pip_install_and_link buildpath`

- only document HOMEBREW_* variables in the manpage (although still
  read from all the original environment variables).
- resort manpage environment variables in alphabetical order
- check the original path for trailing slashes

- Note that Homebrew Cask issues should be filed elsewhere.
- Make `brew`/homebrew-core issue differential in simpler language
- Tell people to fix all `brew doctor` issues
- Refer to the issue checklist and make it even more clear and prominent
  that we may close it out immediately if they don't use it.
- Note that proposed new issues should be widely relevant.
- Don't tell people they need to `brew cask install java`; we handle
  this in most places with a Java requirement and probably want issues
  filed where we don't.
- Remove reference to Homebrew Dupes and replace with reference to
  Homebrew Cask to try and get people filing stuff there instead.

Fixes #2608