Replace api coverage script
- [ ] closes #xxxx
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR adds to the output of the script `validate_docstrings.py` the methods from `Series`, `DataFrame` and `Panel` that are not present in `doc/api.rst`. This makes the script `api_rst_coverage.py` obsolete, as its only functionality was to detect these methods, and the methods in `doc/api.rst` that didn't exist, which was already returned by `validate_docstrings.py`.

Besides that, the PR implements to minor changes. It adds new columns 'section' and 'subsection' to the csv returned by `validate_docstrings.py`, so the csv contains the section and subsection in `api.rst` in which each docstring is defined (besides giving context, this will be useful to divide the docstrings in similar groups for the sprint).

Also, this PR adds some extra validations, like making sure the docstrings starts in the next line after the opening triple quotes, and similar validations.

CC @jorisvandenbossche 