NCTools documentation update for 2023.02

[ngs333]

This PR updates the NCTools documentation by:

1. Slight improvements to the README.md file.
2. Updating the example "extreme frgrid" runscript to work on Gaea C5, and to run 5x faster that the
   prior one with similar amount of resources.
3. provides an outline one part of one fregrid remapping algorithm to highlight the use of some options (e.g.
   cell_measures and cell_methods)
4. Provides a table/document of certain NCTools CI tests that are essentially showing a "workflow" and can provide the
   users with some context.
5. Adds a document concerning a trick for additional testing.

(NOTE:
Some files are being saved in the docs directory as pdf files. The source files for these are these google docs
which should have been shared with all:
https://docs.google.com/document/d/1bHB-_6HKofVTOT0xaX1MRZL_fUKKCxiuOL--zuDyymk
https://docs.google.com/document/d/17IgBJksVO297WAdMP_NRGh5i63BOADbUJ2U6HsE45NI/
https://docs.google.com/document/d/18a8_10_Y_3BGiPoVyYvbKkEUaj62KHsg9EBnWzQOMq0
)

[ngs333]

As per idea of @ceblanton , modified the README.md to mention fregrid_parallel, and also updated the
"extreme fregrid" documentation to clarify the purpose of fregrid_parallel,