x/pkgsite: document links extraction feature

[seankhliao]

What is the URL of the page with the issue?

https://pkg.go.dev/modernc.org/fsm@v1.1.2

What did you do?

See a Links section in the package godoc https://gitlab.com/cznic/fsm/-/blob/v1.1.2/nfa.go#L20

What did you expect to see?

Links section in the rendered documentation

What did you see instead?

No links and no sidebar

Additional Info

This was reported in slack: https://gophers.slack.com/archives/C0166L4QGJV/p1682761965985609

CL 274958 added support for extracting links to show in the sidebar as part of #42968 .
But it's not really documented anywhere
and it silently drops the entire section if the format doesn't match

[dolmen]

Here is the section of the documentation that should be enhanced: https://go.dev/doc/comment#links

[dolmen]

@seankhliao The list of links should probably not be indented.