Advanced Search for Docs

[dsanders11]

Add search for just the documentation quick an inline command button and a QuickPick for search input. Open the search results in a Webview, or just as results in the QuickPick if that can show enough text and be functional enough.

[dsanders11]

Quick search landed in b61985f but is a little stunted by a bug in VS Code's markdown preview not jumping to the header. The quick search only provides linkables as results.

Changing this issue to track an "advanced search" which would search the actual content of the docs and display them in a Webview. So mostly the work here would be for the Webview UI.