docs: add extensibility and forward compatibility guidelines

[gsmith85]

Description

• Added "Extensibility and Forward Compatibility" section to the schema authoring guide.
• Defined standards for Open vs. Closed Enumerations to prevent breaking changes in code-generated clients.
• Added guidance on avoiding 'additionalProperties: false' to ensure forward compatibility for objects.
• Clarified the impact of modern code generators (e.g., Quicktype) on schema-to-type validation.
• Updated the comprehensive capability schema example to align with these new extensibility rules.

Re: #244 (comment), 040684c

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules