Add instructions for converting Markdown to reStructuredText and vice versa

[samsrabin]

E.g.:

pandoc --wrap=preserve --from=markdown --to=rst --output=doc/source/users_guide/working-with-documentation/building-docs-multiple-versions.rst doc/source/users_guide/working-with-documentation/building-docs-multiple-versions.md

[slevis-lmwg]

@samsrabin thank you for suggesting pandoc. Do I need my own download, or is it available somewhere already for us to use?

[slevis-lmwg]

I may have answered my question by downloading pandoc to my Windows laptop and running at the command line. If it worked I will document a few more details here.

[slevis-lmwg]

I'm pleased with the result in #3816 from using pandoc on my Windows laptop, so here's a checklist of what I did:

1. From https://pandoc.org/installing.html I clicked https://github.com/jgm/pandoc/releases/latest that took me to https://github.com/jgm/pandoc/releases/tag/3.9.0.2 where I clicked pandoc-3.9.0.2-windows-x86_64.zip
2. The last click downloaded the zipped folder pandoc-3.9.0.2-windows-x86_64 to my Windows laptop's Downloads. I double-clicked to enter the folder. I copied - by dragging - the contents (i.e. folder pandoc-3.9.0.2) to my Windows laptop's Desktop, thus making the folder visible from the command line of MobaXterm.
3. cd /home/slevis/Desktop/pandoc-3.9.0.2
4. scp slevis@derecho.hpc.ucar.edu:/glade/work/slevis/git/b4b-dev/README_GITFLEXIMOD.rst .
5. ./pandoc.exe --wrap=preserve --from=rst --to=markdown --output=README_GITFLEXIMOD.md README_GITFLEXIMOD.rst
6. scp README_GITFLEXIMOD.md slevis@derecho.hpc.ucar.edu:/glade/work/slevis/git/b4b-dev/.

For the next file conversion I can skip steps 1 and 2.