x/pkgsite: add redirect link to Sourcegraph for method caller / uses of a particular type

[joshuabezaleel]

What is the URL of the page with the issue?

Every package detail page. I first noticed it when accessed https://godoc.org/golang.org/x/tools/go/ssa#FieldAddr

What is your user agent?

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.108 Safari/537.36

Screenshot

What did you do?

Hover to the right of the type/function.

What did you expect to see?

See a "Uses" link that list the method callers for a function and the one who use the type for a type which will then redirects to Sourcegraph for that particular function/type. For example, function Common() will redirect to this link.

What did you see instead?

I saw both the pilcrow (¶) for the type/function link and the Uses redirect link at the old godoc.org, but only saw pilcrow for every package at the new pkg.go.dev.

[joshuabezaleel]

I am not that experienced to Go and pretty new to the codebase but if no one is interested I think I can give this issue a shot 🙂 cc: @julieqiu

[julieqiu]

@joshuabezaleel - feel free to send a CL!

Some information to help you get started with this issue:

1. Check out CONTRIBUTING.md.
2. The documentation HTML is defined in internal/fetch/dochtml/template.go.