feat: Allow packages to define custom documentation URL and badge

[m5ka]

Background

Sally works great in the typical Go documentation ecosystem but when deviating from this it becomes difficult for the documentation badge/URL to be useful, even when specifying a custom godoc.host config variable.

This pull request aims to give a little more freedom with defining the documentation URL and badge image for different packages.

Changes

• Adds doc_url and doc_badge variables to YAML documentation.
• Honours the doc_url variable when the documentation URL is used in templates, though still generating the https://{godoc_host}/{module} URL as a default.
• Honours the doc_badge variable as the image to use for a package's badge on the index page, though still generating the //pkg.go.dev/badge/{module}.svg URL as a default.
• Adds tests for the above two changes (in handler_test.go).

This is a non-breaking feature (semver minor) change. Existing sally setups will continue to work as expected due to these defaults.

[CLAassistant]

All committers have signed the CLA.

[JacobOaks]

Thanks for the PR! I think this LGTM, but I'll wait for another reviewer since I'm newer to Sally.