Add checker instructions into Read the Docs build

[terriko]

Right now, the checker instructions live in the checkers readme file: https://github.com/intel/cve-bin-tool/blob/main/cve_bin_tool/checkers/README.md

I think it would be nice if they were built and linked alongside the other docs on read the docs: https://cve-bin-tool.readthedocs.io/en/latest/

To do this, you'd probably want to add them to https://github.com/intel/cve-bin-tool/blob/main/doc/index.rst and possibly do similar symlink magic we've done with the main README.md file to make it work (your simlink may need to be called CHECKERS.md instead of README.md to avoid conflicts). Then build the docs (instructions) and double-check that it's showing up correctly and nothing looks too confusing in the generated indices.