Documentation updates for develop

This issue will track the sections and topics that need updating when time and resources allow:

[x] Review the document to standardize the use of regular font, italics, and this font
[x] Expand glossary, standardize how glossary items are linked
- [x] Only link a term once per chapter, at that term's first mention in the chapter
- [x] If a term already appears in another linked/defined term (e.g. "CCPP" as a part of "CCPP Physics", do not link in that chapter
- [x] Exceptions can be made for sections that are likely to be linked directly, such as Troubleshooting in CCPPPreBuild.rst
- [x] If a term appears in the glossary but there is a chapter or section describing it in detail, provide a brief description in the glossary with a link to the relevant chapter/section for more information
[x] Section 6.3 in the technical documentation describes CCPP Variables in the SCM and UFS Atmosphere Host Models (https://ccpp-techdoc.readthedocs.io/en/latest/HostSideCoding.html). There was a set of PRs for the ufs-weather-model that modified where these data structures are stored and how they are named (see https://github.com/ufs-community/ufs-weather-model/pull/1130 for the top-level ufs-weather-model PR). This requires updates to section 6.3.
[x] Update README file to remove "GMTB"
[ ] Consider adding information about tests available
[x] Chapter 7, CCPP code management. Should we say something about UFS Fork?
[x] Remove mentions of forum, add mentions of GitHub discussions
[x] Remove unused acronyms from the list of acronyms, such as NEMSfv3gfs

Please edit this issue to add topics as you see fit.

NCAR / ccpp-doc