Add regression testing

[pepopowitz]

We should add some level of regression testing.

• 1087 suggests that, assuming we provided clear feedback about why regression tests are failing, engineers would not be strongly opposed to it.

• 1101 illustrates that some level of regression testing could help prevent bugs from impacting main functions of the site (in that example, website navigation)

• https://github.com/camunda/camunda-platform-docs/pull/1087#issuecomment-1196062011 illustrates a use case for testing popular content
• https://github.com/camunda/camunda-platform-docs/pull/1243#issuecomment-1240884780 illustrates a use case for testing regressions of MDX features
• https://github.com/camunda/camunda-platform-docs/pull/1270 illustrates a case for testing regressions of mixpanel integration, which can currently only be tested in production. In that example, the feedback loop to find the regression was 6 weeks.
• https://github.com/camunda/product-hub/issues/473 illustrates a case for testing regressions of search results. We shouldn't write tests to comprehensively cover all search results, but we should at least write one test to confirm that search results are coming up, and they match our expectations.

My personal takeaways from #1087:

• People aren't begging for regression testing, but they also don't seem opposed to adding it as part of the workflow
• It seems like a straightforward implementation of tests in CI, without a fancy service, would at least initially meet our needs

### Progress
- [ ] https://github.com/camunda/camunda-platform-docs/pull/1547
- [ ] #2524 
- [ ] Regression test: website navigation
- [ ] Regression test: MDX features
- [ ] Regression test: mixpanel integration
- [ ] Regression test: search results

Implementation notes

It's likely that we'll only want to run regression testing if the "infrastructure" of our docs change, as opposed to the content. To handle this, it looks like workflows and/or jobs can be filtered based on which files have changed.

[akeller]

Moving to ready for Q4.

[pepopowitz]

Note for myself: trigger another workflow upon completion success. (If we decide to trigger the regression testing workflow to run only after a production deploy.)

[akeller]

@pepopowitz thinking about our focus in Q1 - can you provide a status update for this card and then move it to the most appropriate column?