Simplify method for generating docs for multiple versions

[rob-luke]

Currently, we build the docs for all version when code is pushed to main. This has several issues:

1. It requires that the same dependency versions work with all past and future MNE-NIRS versions. This is a problem as we drop support for old versions of dependencies, or the dependency APIs change (see Use specific nilearn version for building all docs #434)
2. It is slow to rebuild all versions of the docs, and uses CI resources

@larsoner describes in #434 (comment) how MNE-Python handles multiple doc versions. MNE-NIRS should do something similar.