Update forum link

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Update forum link to https://bb.cgd.ucar.edu/cesm/forums/cice-consortium.146/
• Developer(s):
  apcraig
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  no testing done, only changes documentation md files
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • 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/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.

xenforo forum link was not robust, updated to https://bb.cgd.ucar.edu/cesm/forums/cice-consortium.146/

Closes #564

[apcraig]

For some reason, readthedocs is not able to send a success signal to github. I have verified the documentation builds, so will merge and fix the github/readthedocs issue separately.