Remove outdated code references from extension documentation

[SoryRawyer]

The documentation on writing extensions references a few outdated parts of the codebase. One example is the menu bar section, which references a JS file that no longer exists.

[tfmorris]

This is related to #515 which is also about dead links in the extension documentation.

[nishitagajraj]

I just had a PR merged fixing dead links (#549) and I'd like to tackle this one next. I'll go through the extension documentation and remove/update the outdated references. Can I be assigned?

[tfmorris]

I just had a PR merged fixing dead links (#549)

That PR is not merged. Please work on one PR at a time. This issue is a subset of #515, so may be easier to work on first, but I'll you and @magdmartin decide.