Add documenation for DEPHY converter and NSST behavior

[hertneky]

SOURCE: Tracy Hertneky/Grant Firl

DESCRIPTION OF CHANGES:

• Documentation changes for the SCM only
• Add documentation for the script that converts the legacy format to DEPHY format
• Add documentation the behavior of NSST in the SCM
• RTD link: https://ccpp-scm-hertneky.readthedocs.io/en/548_doc_updates/

ISSUE: #548

ASSOCIATED PRs:
None

[scrasmussen]

Built and read through the changes and they look good!

[ligiabernardet]

Need to change references to the version number, e.g., "The CCPP SCM v7.0.0..." and "git clone --recursive -b v7.0.0"

[grantfirl]

LGTM

[hertneky]

@grantfirl @scrasmussen @dustinswales Does anyone want to re-review before I merge this? Changes since last review were adding the release notes. Otherwise I will merge it this afternoon