feat: show markdown help for missing token

Adds 'Show Help' to the error dialog that is shown when gitlab_service cannot find a token.

Clicking 'Show Help' opens a markdown preview of docs/user/access-token.md:

video

This is done by un-excluding docs/user/** - so that user docs are included in the published extension - and using context.extensionUri to get a URI for the extension root, which can be used to open packaged files.