feat: Show stdlib xrefs as links to official docs

[zsol]

Stack from ghstack (oldest at bottom):

• 114

• -> #113

• 112

When an xref's project field is __std__, the frontend now displays a link to docs.python.org.

Some caveats in this implementation:

• Only the latest version of Python 3 docs is shown.
• Links to class fields (methods, variables) are broken.

Fixes #99.