Spellcheck
This PR is a continuation of my read-through of the docs, the earlier PRs are #18941, #18973 and #18948.

**The changes included in this PR include**
- Changed spelling of "python" to "Python" for consistency, same with "numpy" to "NumPy". This change was made globally, i.e. for most documents in `/docs/source/`.
- A review of `groupby.rst` and `missing_data.rst`
  - Added punctuation where missing.
  - Added more function references (i.e. `:meth:...`) where appropriate.
  - Rewrote a few sentences.

Feedback is very welcome.

