Improving findability of "help my checkers aren't loading" doc

[terriko]

Regarding this section of the docs:
https://cve-bin-tool.readthedocs.io/en/latest/CONTRIBUTING.html#help-my-checkers-aren-t-loading

If you look at the index for our docs, you'll see that this section doesn't show in the index:
https://cve-bin-tool.readthedocs.io/en/latest/index.html

That's because we only show headers at the level above that one. But the question about checker loading has come up a few times and I think we should probably make it more visible. This can be done by changing the number of "#" characters in CONTRIBUTING.md (and don't forget to update the table of contents in the doc; I use a Visual Studio Code plugin to do that but it can also be done manually), or it could be done by changing the table of contents depth in doc/index.rst

I'm not sure offhand which one will look better, so feel free to try both and choose one for a PR. You can build the docs by going into the doc/ folder and typing make, though you may need to install dependencies first (they're in doc/requirements.txt).

[terriko]

Also: If you've already had a pull request merged, please leave this one for a new contributor! (It's really not urgent and is perfect for someone who wants to learn how to build our docs.)