Encrypt session cache

[hawkeyetwolf]

Summary

Authenticate Terminus in a GitHub Actions Pipeline - Replaces session cache with encrypted session cache. It would be great to transfer this action to the Pantheon GitHub org and then publish it to the GitHub Marketplace, but okay to leave under the Lullabot org for now.

Effect

The following changes are already committed:

• A new GitHub Action (https://github.com/Lullabot/terminus-auth-with-session-cache/) to handle saving and restoring an encrypted session from cache.

Remaining Work and Prerequisites

The following changes still need to be completed:

• [ ] Pantheon engineering review of the docs changes (this PR)
• [ ] Pantheon engineering review of the action referenced in the docs: https://github.com/Lullabot/terminus-auth-with-session-cache

Dependencies and Timing

• [ ] Consider transferring action from the Lullabot GitHub org to Pantheon's and update this PR before merging

Release:

• [x] When ready
• [x] ~~After date: $DATE~~

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Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)
• [ ] For Heroes - add a props post to the discussion board.
• [ ] Remove from the project board