redoc-cli: test script to run on user-supplied openapi spec

[IvanGoncharov]

It would be useful to have script or tool to run in browser tests on user's Swagger spec.
It could be as simple as running a headless browser and check that there is no errors or warnings.
Would be the ideal test to integrate into CI just before deploying Swagger spec.
Important thing is to output banner which explains that this errors could be unrelated to Swagger spec but purely ReDoc issues and if so ask user to open issue in ReDoc repo.

[RomanHotsiy]

accidentally closed by wrong commit-message

[lornajane]

Closing this since redoc-cli is deprecated. Users should migrate to Redocly CLI.