Use extra.version if it is available when generating source partial

[brendo]

A very small change remove the hardcoded version in source.html. It now uses extra.version set in the mkdocs.yml config.

In future, it may be possible to retrieve this information from the Github API along with the stargazers and fork counts.

[squidfunk]

I was thinking of making this entirely dynamic, because it makes no real sense when we are loading changes from the GitHub API anyway. There are already traces of some prototyping in application.js which I have to refactor.

[brendo]

That's true, but for Gitlab and Bitbucket sources it will be difficult to retrieve that data as they don't expose public API's in the same way that Github does.

[squidfunk]

Okay, I will merge it and check the GitLab and BitBucket APIs later. Thanks for your contribution!