Add Schema documentation

[dakota002]

Acceptance Criteria

Schema documentation pages exist for producers and consumers of notices:

• Gitflow instructions for PRs and schema development
• Instructions on how to create multiresolution healpix maps
• Instructions for constructing schema from Core Schema objects and additional instrument specific
• instructions on how to base64 encode fits files and include them in a JSON notice
• Instructions for decoding base64 encoded fits file string (w/ example)
• Best practices for schema updates (ex. Deprecation of fields that are being updated or replaced,)
• Details of allOf and anyOf; when and why to use them
• "unevaluatedProperties": false, Validator does not validate when there are multiple subschema references, fails when setting "unevaluatedProperties": false gcn-schema#80
• Code for both schema and example: Strong starting point and min confusion

[lpsinger]

• Instructions on how to create multiresolution healpix maps

See follow-up issue: #1493.

[Vidushi-GitHub]

Already an issue exists for it, see #1205, although needs re-base now.

[Vidushi-GitHub]

Thanks @lpsinger, for completion of final deployment. @jracusin this should be good for making announcement.