Missing documentation for default-language and other-languages

#### Describe the bug

I expected to see the documentation for `default-language` and `other-languages` fields in the [build information](https://cabal.readthedocs.io/en/latest/cabal-package.html?highlight=default-language#build-information) section of the docs. However, it is not present there. Searching the documentation doesn't help as well.

I found the syntax documentation in "Field Syntax Reference" [section for build-info](https://cabal.readthedocs.io/en/latest/buildinfo-fields-reference.html?highlight=default-language#build-info-fields). But the links for the corresponding documentation are missing there, even though the section explicitly says "Documentation for `default-language`".

#### Expected behavior
I expect to see the documentation for `default-language` and `other-languages` fields in the build information section of Cabal docs.

#### Additional context
Also, I expect to have the documented default used for `default-language` when this particular field is missing in the `.cabal` file. Is it `Haskell98` or `Haskell2010`?

Start with the Build Information section and the Field Syntax Reference for build-info, linked in the issue. Add documentation for `default-language` and `other-languages`, connect the existing field references to it, and clarify the default used when `default-language` is absent. Done means both fields are searchable and documented consistently.

Written by the indexing model from the issue text.