Add resources for LayoutLmV2 and reformat documentation resources
# What does this PR do?

From #19848 
This PR adds resources to the LayoutLMV2 documentation page.

Also in the LayoutLMV2 documentation page the documentation resources heading was incoherent with the other doc pages so I removed the heading and put the task specific guides under the corresponding task headings. 


cc @stevhliu <|||||>_The documentation is not available anymore as the PR was closed or merged._