gopls/doc: generate documentation for analyzers

Analyzers are configured in the internal/lsp/source/options.go file as
well as settings, and we can generate documentation for them without
even using reflection. We add documentaton for each analyzer and list
whether or not it is enabled by default.

Change-Id: If0ffcd422f3f4a99ca3645c35197925ea1cc1616
Reviewed-on: https://go-review.googlesource.com/c/tools/+/280352
Trust: Rebecca Stambler <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>

Author: stamblerre