feat(metamodel) document considerations for metamodel extensions

[dselman]

Adds a new reference doc page that describes considerations that go into metamodel extensions and the changes that need to be made.

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Author Checklist

• [ ] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [ ] Vital features and changes captured in unit and/or integration tests
• [ ] Commits messages follow AP format
• [ ] Extend the documentation, if necessary
• [ ] Merging to master from fork:branchname
• [ ] Manual accessibility test performed
  • [ ] Keyboard-only access, including forms
  • [ ] Contrast at least WCAG Level A
  • [ ] Appropriate labels, alt text, and instructions

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