Fix RTD, minor updates to documentation

[apcraig]

PR checklist

• [X] Short (1 sentence) summary of your PR: Fix Readthdocs after recent changes, update documentation

• [X] Developer(s): apcraig

• [X] Suggest PR reviewers from list in the column to the right.

• [X] Please copy the PR test results link or provide a summary of testing completed below. All bit-for-bit in full icepack test suite, https://github.com/CICE-Consortium/Test-Results/wiki/icepack_by_hash_forks#f36fccbddd20ef64bb95282dd85db726dbaa1f76. PGI tests did not complete due to login timeout, should be OK. CCSMCOUPLED cpp still needs testing in coupled system.

• How much do the PR code changes differ from the unmodified code?

  • [X] bit for bit
  • [ ] different at roundoff level
  • [ ] more substantial

• Does this PR create or have dependencies on CICE or any other models?

  • [ ] Yes
  • [X] No

• Does this PR add any new test cases?

  • [ ] Yes
  • [X] No

• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)

  • [X] Yes
  • [ ] No, does the documentation need to be updated at a later time?
    • [ ] Yes
    • [ ] No

• [X] Please provide any additional information or relevant details below:

• Update readthedocs implementation consistent with @dabail10 implementation in https://github.com/CICE-Consortium/CICE/pull/827.

• Update icepack interface documentation and add new public mushy subroutines (should have been done before).

• Remove trailing spaces in source code.

• Add cpp CCSMCOUPLED equivalent to CESMCOUPLED via icepack_kinds.