Add former subpackage_index.html

[regisss]

What does this PR do?

This PR generates a {subpackage}_index.html so that existing links to it still work. They currently don't work because of the doc overhaul (see #429).

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lewtun]

Thanks for the fix @regisss ! I checked the tiles links on both the copy & source index.html file and they look fine to me.