feat!: search improvements

[frarees]

Motivation

DocFX's current state of the search functionality has some flaws:

• Searching through APIs can yield no results unless you type the complete symbol name
  • For example, if I have a page ManagedReferenceExample and I search for Managed I will get no results. Same goes for ManagedReference
  • This is specially important for API pages, since one of the reasons users want to use the search functionality is because they've forgotten the full name of a member
• Searching multiple terms doesn't narrow the search, but the opposite
• When I finish inputting my search query, I press Enter instinctively, which clears the search and reloads the page

What has changed

• New global metadata keys _searchIndexUseMetadata and _searchIndexUseMetadataTitle to opt-in
• New build step for MREFs: FillMetadata
  • Stores Title and Summary in ManifestProperties
• Search Index
  • Includes new property summary
  • Entries for MREFs can be loaded from metadata instead of HTML scrapping
• SearchIndexItem.Keywords holds different content for MREFs
  • Parses through the symbol's full name and creates permutations of the words found
    • ManagedReferenceExample -> Managed ManagedReference ManagedReferenceExample ManagedExample Reference ReferenceExample Example
  • Keywords is still present and it's used to store exclusively for keywords, and processed by the search engine
  • Summary is rendered by the themes (brief)
• Modern theme
  • Don't allow Enter/Submit on search
  • Search is additive: searching multiple terms uses AND instead of OR

Notes / TODO

• The regex used to create keywords, while accounting for most cases, is not perfect
• Default theme could use similar changes to modern theme, so UX is similar

[cdervout]

Hello
Any news about this PR ?

[frarees]

I'm done on my side (with some minor tweaks that could be done, see TODO).
Awaiting team's review.

I will push a merge against current main to keep it in sync.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.36%. Comparing base (fe673ec) to head (3e393a4).
Report is 499 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/Docfx.Build/PostProcessors/ExtractSearchIndex.cs	88.63%	1 Missing and 4 partials ⚠️
src/Docfx.Build/PostProcessors/SearchIndexItem.cs	44.44%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10113      +/-   ##
==========================================
+ Coverage   74.31%   79.36%   +5.04%     
==========================================
  Files         536      548      +12     
  Lines       23189    23686     +497     
  Branches     4056     4079      +23     
==========================================
+ Hits        17234    18799    +1565     
+ Misses       4853     3713    -1140     
- Partials     1102     1174      +72     

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