Browser tests

[jcreedcmu]

Create some sort of Playwright/Selenium/Cypress test setup so that we can check that rendered verso documents have expected end-to-end properties like expected content, javascript interactions, etc.

Example documents we might like to check are

• the verso users' guide
• any other purely-for-the-purposes-of-this-kind-of-testing verso documents that might exist in the verso repo
• test deployments of reference manual PRs
• https://lean-lang.org/doc/reference/latest/

This would have been useful for catching https://github.com/leanprover/verso/issues/555 earlier.

[david-christiansen]

We now have Playwright, and it checks that the redirects work. I think we should close this now.

[robsimmons]

I will leave this open as an add-a-few-more-checks note