[Docs] Minor fixes
# What does this PR do?

This PR:
- moves TAPAS and LayoutLM to the "multimodal" section rather than "text", as these models leverage more modalities than just text (TAPAS leverages row and column information, LayoutLM leverages 2D coordinates).
- adds a figure for DETA and fixes the one of UPerNet
_The documentation is not available anymore as the PR was closed or merged._