First pass at documenting maskbits

[forero]

First version for the maskbits documentation. It simply links to the source code on desihub where the meaning for each bit in the mask is explained.

[coveralls]

Coverage: 100.0%. Remained the same when pulling f52e062 on maskbits into 890e960 on main.

[weaverba137]

Looks good so far, with some minor changes. I think it is an open question though whether we want the general public digging down into the YAML files for the individual bits.

[weaverba137]

@forero, we should revisit this in preparation for EDR. There are still some review comments to be addressed.

[forero]

Hi @weaverba137. I think I have addressed the two issues that you raised. Please let me know what you think.

[weaverba137]

Thank you looks good.