Add menu item for "Browse gopls feature documentation"

[adonovan]

The gopls.doc.features code action (requires gopls at tip) opens the gopls manual in a browser. However, this code action appears only when the "command-." key is pressed to open a menu of miscellaneous code actions. It should be a more prominent command in the UI: directly available in the command palette and the toplevel context menu.

xtools$ gopls codeaction ./gopls/main.go 
command	"Browse gopls feature documentation" [gopls.doc.features]
command	"Browse documentation for package main" [source.doc]
xtools$ gopls codeaction -exec -kind=gopls ./gopls/main.go 
(opens browser)

Also: we observed that this code action didn't appear at all for one user (@hyangah). We should investigate.