Extra docs linting: ensure that documents referenced in extra-docs.yml are actually there

[felixfontein]

Ref: ansible-collections/community.general#10239 (comment)

[felixfontein]

Apparently toctrees in extra docs RST files can also have invalid entries; rstcheck won't notice since it considers one file at a time. I guess we'll have to use docutils to look at all toctree environments (similar to how we look at all code blocks)...

[felixfontein]

Fixed by #399.