Missing `GL06` and `GL07` errors for invalid docstrings
As discovered in the process of working on #26301, there are invalid docstrings that should throw either `GL06 - Found unknown section` or `GL07 - Sections are in the wrong order` errors.

Without too much investigation, it's not clear why `GL06` is not being thrown. Currently unsure if this is even a problem because, as @datapythonista mentioned in comments on #26301, we may not even run these checks on the private methods.

The missing `GL07` are because section headers were only considered section headers if the following line had an equal number of `-----`'s (underlines).

This code is found here (where `content[0]` is the section header):
https://github.com/pandas-dev/pandas/blob/2bbc0c2c198374546408cb15fff447c1e306f99f/scripts/validate_docstrings.py#L383-L386

- [x] Reorder sections that are failing #26301 and merge
- [ ] ~Fix GL07 to correctly throw errors with invalid underlines~
- [ ] ~Investigate GL06~
- [ ] Add error for valid section title words with invalid number of underlines

If it's okay with others, I'd like to take a crack at this issue.

