[acc] Fix OpenACC documentation (#77502)

After PR#75548, the OpenACC documentation on the MLIR website has a few
issues. This change corrects them:
- Renames OpenACC.md to OpenACCDialect.md so that links remain
unchanged. In its current state, the links to
https://mlir.llvm.org/docs/Dialects/OpenACCDialect/ no longer work.
- Since the old OpenACCDialect.md (the one with operation definitions)
is being included in the new file, rename the old file to prevent name
ambiguity.
- A header is needed in the .md file, otherwise the index on website is
not properly created.
- Add a new section before including the operations .md file because
otherwise the separation is not clear.