DOC: Spellcheck of merging.rst, reshaping.rst and timeseries.rst
This PR is a continuation of my reading through the docs, see #19017, #18973, #18948, #18941.

The changes are:
* In `merging.rst`, `reshaping.rst` and `timeseries.rst`:
  * Function references as links
  * Backticks ` `` ` around Series, DataFrame
  * Minor rephrasing of sentences, spelling, etc
* Replaced some instances of `python` with `Python` and `numpy` with `NumPy` globally in `*.rst` files. I hope to add a lint rule in a future PR.

Comments/reviews are very welcome.