docs: various updates

[jonchang]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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• analytics: linkify links to Google documentation, clarify current usage, policy, and first-run behavior
• brief mention about using custom Requirements
• update current practice w.r.t. cross-compilers and alternative toolchains
• FAQ: move terminology question to the top (since we get complaints about it all the time), remove deprecated behavior (manual brew cleanup, brew pull, etc.), include new features in brew info and brew extract for disabled/deleted formulae
• livecheck is no longer an external command
• remove outdated mentions of npm@5 and python 2.7
• include relevant mentions of the Homebrew discussion forums where needed
• brews -> formulae
• clarify that we don't permit tap migrations out of homebrew/core
• move kickstarter supporters to its own bullet point group

[issyl0]

This is really comprehensive, thanks Jon! 👏🏻

[MikeMcQuaid]

Nice cleanup, thanks!

[jonchang]

Thanks for the reviews all!