[docs] NIX file validity and rules for validator

[achilleas-k]

I was working on making sure the rules for the NIXPy validator match the one in this library and while reading through the validator code, writing down all the rules, I figured I might as well make available for documentation purposes.

I can imagine several target users for this document:

• Contributors/developers who don't want to violate any of the detailed rules of the NIX format.
• Users that might want be wondering why or how their NIX files are invalid (according to the validator).
• Me! (it's mostly for me).

This was written based entirely on the validator code, so if there's anything that's wrong, it might also be a mistake in the validator itself. There might also be some things that aren't expressed clearly, so let me know what you think.

Also not that I haven't linked to this file from anywhere else in the docs, so let me know if you want me to reference it from any of the other documents.

[jgrewe]

one quick comment even without reading: Thank you! The other thing is that you used markdown, but if we wanted to include it in the read-the-docs, we should use rst instead. I always wanted to remove the md files but kept them for reference ...

[codecov[bot]]

Codecov Report

Merging #791 into master will not change coverage. The diff coverage is n/a.

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  Coverage   95.34%   95.34%           
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  Files         197      197           
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  Hits        11644    11644           
  Misses        568      568

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[gicmo]

Nice!