Prepare 3.2.0 and 3.1.2 releases

[lornajane]

Creating this tracking issue for the tasks that need doing around the release.

These are the pull requests we won't release without (other PRs may still merge via the normal processes, these are must-haves)

• v3.2: $self field (Alternative Approach) #4556
• v3.2: Support ordered multipart including streaming #4589
• v3.2: Decide on names of new Example Object fields #4658
• v3.2: How to use style with multipart (and headers/cookies?) [percent-encoding issues] #4798
• v3.1: application/x-www-form-urlencoded potential inconsistency between examples and referenced RFC #4813