[doc] :obj: hunt
I think it'd be easier to review if we focus on one type at a time. So this PR is about `obj`:

it catches all the invalid `:obj:` versions, reformats the next field as if they were normal and then removes them all.

```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's#:?obj: ?\*([^\*]+)\*#`$1`#g' {} \;
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's#:?obj: ?##g' {} \;
git checkout utils/check_repo.py src/transformers/tokenization_utils_base.py src/transformers/models/marian/convert_marian_tatoeba_to_pytorch.py
git checkout examples
make fixup
```
mods reset for `research_projects` as requested, but it should be easy to replay these later.

@sgugger
Redone w/o examples and utils