docs: Convert REQUIREMENTS file from HTML to Markdown

[echoix]

This replaces #2726 and is a followup of #2729 that was just merged.
Closes #2726

[echoix]

@nilason Could you set this PR to close another PR, the "closes" keywords don't seem to work for a PR.

[echoix]

I think we can open an issue to talk about improving the contents of the page. For now, it only needs to be merged since the current file isn't Markdown yet.

[nilason]

@nilason Could you set this PR to close another PR, the "closes" keywords don't seem to work for a PR.

It had to be done manually.

[echoix]

@nilason Could you set this PR to close another PR, the "closes" keywords don't seem to work for a PR.

It had to be done manually.

Thanks!

[nilason]

@echoix Thanks for your contribution and patience!

[echoix]

You're welcome! I didn't realise that the project was finalizing its release in the beginning of January, so waiting for the new merge window is totally correct once understood ;)