Add codeautolink_warn_on_no_backreference config

[cmarqu]

Highlights where reference documentation does not have a corresponding tutorial or how-to

Closes #161

• Tests written and passed
• Documentation and changelog entry written, docs build passed
• All tox checks passed

[cmarqu]

@felix-hilden I could not easily figure out how a test would need to be written. Feel free to push to this PR directly.

[felix-hilden]

I think we should be able to write a test where we inject the refs but have no code examples. For example: https://github.com/felix-hilden/sphinx-codeautolink/blob/master/tests/extension/fail/missing_inventory.txt

[cmarqu]

I think we should be able to write a test where we inject the refs but have no code examples. For example: https://github.com/felix-hilden/sphinx-codeautolink/blob/master/tests/extension/fail/missing_inventory.txt

Ah yes, I was wondering how the check is done, but I get it now - because the test is in the fail/ directory, it's automatically expected to fail, and if I only set the codeautolink_warn_on_no_backreference = True config option, I am not accidentally checking the wrong fail.

[felix-hilden]

Code looks good now!

• feel free to add the lint failure to the extend-ignore, seems useless
• please rebase for the release notes
• let's call this 0.17, you've submitted nice additions! 🎉

[cmarqu]

Seems to work fine now.