[docs] Adding instruction to build custom documentation based on version or commit hashes

[gnzng]

This PR addresses issue #42 , it is not solving it completely, but given instructions on how to build documentation of older versions or git commit hashes.

[yoshikisd]

Thanks for making this, I was just wondering about how to build the docs on my machine! The instructions work on my machine.

From the standpoint of consistently organizing the documentation, I feel that it may be more appropriate to include this as a page in the sphinx docs. However, I might be missing something here; is there an advantage to keeping docs/build_custom_docs.md a standalone markdown file?

[gnzng]

Keeping docs/build_custom_docs.md as a standalone markdown file has a few advantages. It allows for quick access and easy reference by developers without needing to navigate the Sphinx documentation (which is good because it is about building the docs). Markdown files are simpler to read and edit directly in a text editor or on github. The instructions remain accessible even if Sphinx is not setup or the hosting is down.

[yoshikisd]

That makes sense. In that case, we should keep this as a markdown file. I'll approve this PR and merge it.

[yoshikisd]

Hmm, looks like the first attempt to run the checks failed with this error being reported:

remote: Internal Server Error
Error: fatal: unable to access 'https://github.com/cdtools-developers/cdtools/': The requested URL returned error: 500

Not sure if this has any relation to the cloudflare outage that happened today. I tried running the tests again and things completed just fine.

[gnzng]

github is having some issues: https://downdetector.com/status/github/