Update docs site for version 2.18.1

[github-actions]

Docs from https://github.com/pantsbuild/pants/releases/tag/release_2.18.1

[huonw]

@thejcannon this is a doc sync PR that I would've expected to have no changes to the generated reference files, now that we've theoretically resolved #37, with:

• Check in help-all.json for posterity #43
• Generate reference docs with all backends enabled #44
• Regenerate reference docs from in-repo sources #50
• Patch holes in reference generation #53

Are you able to take a look and work out what's changed and if it requires updates on our side? For instance, the help-all.json seems to have significant changes, plus a bunch of extra files, and many files with minor changes too.

[thejcannon]

OK I thumbed through a bit, and commented on a few things.

plus a bunch of extra files

Those seem to be from backends that weren't added to the static list of backends to activate on the relevant branch.

Part of this change was eliminating the need for a static list of backends to generate docs for, and instead just generating reference docs for all possible backends.

We can apply all of these fixes/changes using the existing help-all's version to ensure we're minimizing changes and then do another round of this PR (trying to sync with a later version)

[huonw]

Closing and trying again after the next batch of fixes #73, #80, #81 (nice work @thejcannon )

[huonw]

#82