[WIP] Add references to mongo-hibernate

[yrodiere]

What we have at the moment:

1. MongoDB extension in the "About" page for Hibernate ORM: https://staging.hibernate.org/orm/#compatibility-with-a-wide-range-of-databases
2. MongoDB extension in the compatibility matrix: https://staging.hibernate.org/orm/releases/#compatibility-matrix
   Ajay: please note this will need to be updated on every new release of the MongoDB Extension for Hibernate ORM.
3. Changes to the Hibernate ORM docs (will come in a separate PR, since it's a separate repo)

WIP, because we still need to add MongoDB to the Community section (forums, issue trackers, corporate contributors, ...), but we'll need Ajay's input for that.

[atandon2024]

LGTM