x/pkgsite: support rich formatting for vuln details

[hyangah]

Either support a basic markdown or https://go.dev/doc/comment style syntax.
If go/doc/comment is used, both terminal friendly text output & html formatting are supported
(i.e. govulncheck doesn't need to implement its line wrapping logic

At least, URLs need to be linkified.
For example, in https://pkg.go.dev/vuln/GO-2022-0535 the vulnerability description contains a link
but it is presented as a text, so the link is not clickable.

Not sure if this FR belongs to x/vulndb, x/vuln, or x/pkgsite though.

cc @neild who inspired me :-)