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