Fix broken links in Concerto model docs

[shivengoomer]

Closes #431

This PR fixes broken links under the Concerto Model section in the Accord Project documentation. Now, clicking on Vocabulary, Namespaces, Classes, etc., correctly redirects to the relevant documentation pages.

Changes

• Fixed broken links for the following sections:
  • model-vocabulary
  • model-namespaces
  • model-classes
  • model-enums
  • model-properties
  • model-relationships
  • model-decorators
• Ensured all links now lead to the correct documentation pages.

Flags

• Double-check the URLs to ensure all links now work as expected.
• Verify that there are no remaining broken links in related sections.

Screenshots or Video

(Add screenshots or a short screen recording showing the fixed links in action.)

Related Issues

• Issue broken links #431

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.
• Commit messages follow AP format.
• Extend the documentation, if necessary.
• Merging to master from fork:fix-broken-links.
• Manual accessibility test performed:
  • Keyboard-only access, including forms.
  • Contrast at least WCAG Level A.
  • Appropriate labels, alt text, and instructions.

[mttrbrts]

Thanks for opening this issue, and raising a PR.

The failing links should be removed from sidebars.json, as we've now migrated all Concerto documentation to it's own subsite.