Add CircleCI deploy key with write permission

[jdahm]

Uses a new ssh key stored on CircleCI that includes write permission.

Follows https://circleci.com/docs/github-integration/#create-a-github-deploy-key.

[mcgibbon]

Unfortunately this didn't end up testing anything for the docs because the docs were unchanged - can you make a minor edit and commit so the github logic runs?

[jdahm]

Deployed successfully! https://github.com/ai2cm/pace/actions/runs/3744373215