[X] Short (1 sentence) summary of your PR:
Update readthedocs.yaml, turn on pdf
[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.
Confirmed documentation builds including pdf
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/.)
[ ] Yes
[X] No, does the documentation need to be updated at a later time?
[ ] Yes
[X] No
[ ] Please provide any additional information or relevant details below:
PR checklist