x/pkgsite: update retract documentation

[hyangah]

• improve https://pkg.go.dev/about#removing-a-package by adding practical guides like https://play-with-go.dev/retract-module-versions_go119_en/
• Once x/pkgsite: allow excluding a module/package prefix via a CL #59622 is done, also add this option to https://pkg.go.dev/about#removing-a-package
• fix the broken links in the blog post https://go.dev/blog/go116-module-changes#module-retraction
• add link to more examples to https://go.dev/ref/mod#go-mod-file-retract (or create a wiki page that has different cases e.g. cases like x/pkgsite: package removal request for fauna/faunadb-go/v5 #59577)
• update https://github.com/golang/go/blob/master/.github/ISSUE_TEMPLATE/02-pkgsite-removal.md

cc @golang/tools-team