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

Some projects use custom documentation that doesn't generate the objects.inv file. For these projects, cross-linking to their docs is currently not really possible.

Describe the solution you'd like

It would be nice if it was possible to specify some concrete object -> url mappings right from the config e.g.:

plugins:
  - mkdocstrings:
      handlers:
        python:
          import:
            - url: https://docs.python.org/3.13/objects.inv
              domains: [std, py]
            - https://typing-extensions.readthedocs.io/en/latest/objects.inv
            # Proposed new section:
            - objects:
                - httpx.AsyncClient: https://www.python-httpx.org/api/#asyncclient
                - httpx.Client: https://www.python-httpx.org/api/#client

Describe alternatives you've considered

It would probably be possible to write the objects.inv file for the docs of the external project manually and then host is somewhere for mkdocs to pull it. However, the objects.inv format isn't that common and I'm not sure how easy would generating it like this be. The content of this file seems to be compressed, which makes writing it by hand impossible.