DOC: improve shared content between comparison pages
This pull request does a few things between the SAS and Stata pages, in separate commits:

- Makes the headings match, where it makes sense for them to
- Create more shared includes, as a follow-up to https://github.com/pandas-dev/pandas/pull/38887
- Improves some wording and ensures more methods are linked in the comparison includes

The motivation here is that I'm working on adding the other sections to the Comparison to Spreadsheets page, and want to ensure they're consistent.

---

- [ ] ~~closes #xxxx~~
- [x] tests added / passed
- [ ] ~~passes `black pandas`~~
- [ ] ~~passes `git diff upstream/master -u -- "*.py" | flake8 --diff`~~
- [ ] ~~whatsnew entry~~
