Documentation revamp.

[davidmorgan]

For #4119.

Full rewrite of the most obviously user-facing docs with a focus on how people mostly use build_runner today: build_runner, build, build_test. Added a list of popular builders based mostly on download stats from pub.dev.

Tweaks/cleanup but not a rewrite of docs that are not so user-facing: build_config, build_web_compilers.

Removed most other docs and FAQs, a lot of it is out of date and refers to niche uses of build_runner. Moved a few FAQ entries related to build_config into the build_config doc.

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

[jensjoha]

Mentioning specific packages could seem like an endorsement and might not be what we want?
Maybe it would be better if we restrict it to packages we control and/or already endorse somehow?

[davidmorgan]

Mentioning specific packages could seem like an endorsement and might not be what we want? Maybe it would be better if we restrict it to packages we control and/or already endorse somehow?

Thanks! I added a column listing endorsements and a note that we don't endorse if not mentioned, how does that look to you please?