Add Sky spec

[HarelM]

Launch Checklist

This is in continue to the following PR and relevant to the following issue:

• Style Spec for sky and fog #298
• Design proposal: Atmosphere / Sky parameters #163

It adds the relevant type, adds the validation, add the relevant tests and adds the updates the docs.
I have also made some small changes to the docs to improve maintainence.

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (86c4898) 92.09% compared to head (d680596) 92.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #478      +/-   ##
==========================================
+ Coverage   92.09%   92.14%   +0.04%     
==========================================
  Files         100      101       +1     
  Lines        4176     4200      +24     
  Branches     1197     1201       +4     
==========================================
+ Hits         3846     3870      +24     
  Misses        330      330              

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