Doc fixes
# What does this PR do?

This PR adds multiple fixes for the new frontend, mainly:
- remove all the sphinx-specific files
- convert the old index.rst to index.mdx
- adapt the scripts that touched the index.rst (to update the list of models and the table of frameworks)
- fix the image links in the parallelism.md file
- convert the detr.rst file to have the table in MarkDown format.