De-boost reference docs; add search gadgets

[brynpickering]

Fixes #670

Summary of changes in this pull request

• De-ranked API (0.5) / schema (0.25) docs
• Added search suggestions, sharing, and result highlighting, since we can.

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[irm-codebase]

Checked the results in the documentation.
Much better, but there is still quite some spam (the sidebar looks very 'long' for most searches).
I think this happens because mkdocs includes whole notebook sections in the search, even if only a single word was hit.

source_use_equals leads to an artificially large search (the 'ghost' bar in the picture).
I'm ok with the change, but I think it would be improved by completely omitting notebooks.

[irm-codebase]

Also, the new hints and highlights are much appreciated!

[irm-codebase]

Approved since the notebooks / search bar thing is kind of a separate issue.