regression suite for production docs

[Janpot]

Problem:

• docs-only issues not caught by the demo regression tests [TimelineItem] Keep the spacer selector at one class of specificity material-ui#49028 (review)
• app search is regression sensitive and painful to upgrade, see [docs] Bump @docsearch/react to ^4.7.0 material-ui#48886 and [test] Add a visual regression fixture for the DocSearch modal material-ui#49002

Solution:

• screenshot testing suite for production docs. Build and run docs in CI and take real screenshots
• also a few interactions to test search