x/pkgsite: add repository link on documentation page

[tonglil]

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/sirupsen/logrus?tab=doc
https://pkg.go.dev/github.com/sirupsen/logrus@v1.6.0?tab=doc
https://pkg.go.dev/github.com/sirupsen/logrus?tab=doc#AddHook

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:75.0) Gecko/20100101 Firefox/75.0

What is the issue?

There are no links to the github/source repository from the doc pages.
I don't want to click two clicks to go to the source:

What did you expect to see?

1. I want to be able to click on this to go to the source:

This is godoc.org's behavior when you click on the package name:

2. I should also be able to click on any tag version to get to the source's tree of that version:

3. I should be taken to the master branch when i click on the function reference, instead i am taken to a tagged version, even though I am on a "tagless" version of the doc site (https://pkg.go.dev/github.com/sirupsen/logrus?tab=doc#AddHook is not @Version).
   This is godoc.org's behavior.

What did you see instead?

1. I can't click on it, need to copy and paste + hit enter.

2. I can't click on it.

3. I am taken to https://github.com/sirupsen/logrus/blob/v1.6.0/exported.go#L50 instead of https://github.com/sirupsen/logrus/blob/master/exported.go#L50

[tonglil]

Bonus

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