fix(docs): template guide on concerto model broken redirects in official docs.

[Vinyl-Davyl]

Closes #438

Broken Redirects in Concerto Model Section of Tech Docs

Description:

Several redirects in the Concerto Model section of the technical documentation are broken, rendering them inaccessible to users. The following redirects are affected:

• Vocabulary
• Namespaces
• Classes
• Enumerations
• Properties
• Relationships
• Decorators

Users attempting to access these sections for information on the Concerto Model encounter broken redirects, hindering their ability to understand and utilize these essential components effectively.

Changes

For some reasons, we have redirects to the concerto docs on click of items in that section(Template Guides >> Concerto Model) on the tech docs which redirects to an empty page, which probably breaks the general experience and purpose of the doc coupled with this issue.

• So I got the details for each of this missing section and pages from the concerto-docs repository https://github.com/accordproject/concerto-docs/blob/main/docs/design/specification/model-decorators.md
  https://github.com/accordproject/concerto-docs/blob/main/docs/design/specification/model-enums.md
  https://github.com/accordproject/concerto-docs/blob/main/docs/intro.md
  and so on.
• Then updated this properly in as MarkDowns in the Tech Docs.

Screenshots or Video

The following links from the Tech-Docs Currently redirect to a page like this

Under the Template Guides dropdown accordion in the tech-docs,
This happen in the following redirect and supposed links

1. https://docs.accordproject.org/docs/model-vocabulary.html
2. https://concerto.accordproject.org/docs/specification/model-namespaces.html
3. https://concerto.accordproject.org/docs/specification/model-classes.html
4. https://concerto.accordproject.org/docs/specification/model-enums.html
5. https://concerto.accordproject.org/docs/specification/model-properties.html
6. https://concerto.accordproject.org/docs/specification/model-relationships.html
7. https://concerto.accordproject.org/docs/specification/model-decorators.html

Related Issues

• Issue template guide on concerto model broken redirects in official docs #438

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