Add usage examples to Go docs

[beyang]

This adds links to usage examples on Sourcegraph for the Go documentation by adding a new HTML filter for the Go documentation.

This is how they look on devdocs:

Variable examples:

Constant examples:

Function examples:

Constructor function and method examples:

These link to pages like this on Sourcegraph: https://sourcegraph.com/github.com/golang/go/-/land/GoPackage/bufio/-/NewReader.

Unit tests are included in usages_test.rb.

[Thibaut]

None of the links appear to work (404).

[Thibaut]

@beyang ping

[beyang]

Hey @Thibaut, really sorry about the delay. We made some changes on the backend that changed some URL structures. We should have new versions up in the next couple of days. I'll close the PR until then and update it when it's ready.