DP-26913: Fixes running BackstopJS locally in DDEV, updates documentation, and adds a CircleCI job to capture screenshots nightly

[justafish]

Description:

• Adds a DDEV command to allow BackstopJS to be run locally
• Increases shm_size to 2gb to avoid the browser crashing inside a docker container. See https://bugs.chromium.org/p/chromium/issues/detail?id=519952 and https://github.com/SeleniumHQ/docker-selenium#--shm-size2g
• Switches CircleCI to using backstop's provided image
• Updated the documentation and adds instructions for running the backstop job using CircleCI's local CLI
• Adds a CircleCI job which collects Backstop reference images nightly and stores them as an artifact
• Adds an additional backstop test during the deploy_cd CirlceCI job which uses reference images from the above job

Jira: (Skip unless you are MA staff)
DP-26913

To Test:

• Follow the instructions in the README for running things locally
• Verify examples running in CircleCI for the reference and test jobs from earlier in this PR
• Run ddev drush ma:ci:backstop-snapshot prod --ci-branch=dp-26913-run-backstop-js-locally successfully (this might take a few tries as production still has timeout issues trying to gather reference images)
• Run ddev drush ma:ci:backstop-compare --reference=prod --target=test --ci-branch=dp-26913-run-backstop-js-locally to test against the newly generated reference images

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Peer Review Checklist

[weitzman]

PR looks good to me. I'm OK with merging this now, or waiting for the cache bust bypass to be added.