[FEATURE REQ.] Add links between different versions of base-classes form different sources

[calumbell]

Is your feature request related to a problem? Please describe.

While working on the classes/[className] page on the front-end, I was thinking that it would be nice to include a little piece of UI that links to different versions of a base-class from different document sources. ie. It would be nice if the D&D 2024 Fighter linked to the 2014 Fighter and (eventually) the A5e Fighter. Currently, the API doesn't support this kind of opperation.

Describe the solution you'd like
Add a field to the CharacterClass model called "related_classes", "versions", something like that, which contains a list of FKs to different versions of the same class from different document sources. It might look something like:

{
  "name": "Fighter",
  "key": "srd-2014_fighter",
  "versions": [
    {
      "name": "Fighter",
      "key": "srd-2024_fighter",
    },
    {
      "name": "Fighter",
      "key": "a5e_fighter",
    },
  ],
}

Describe alternatives you've considered
I have considered extrapolating this infomation on the front-end by performing a search for classes with the same name, but this won't work with related classes with different names. ie. Barbarian (D&D) -> Berserker (A5e), Paladin (D&D) -> Herald (A5e)