Add OpenAPI workflow to confirm it's working

[nickvergessen]

☑️ Resolves

• CI for Update openapi-extractor to latest #11314

[nickvergessen]

Failing run on stable28 before the merge of #11315
https://github.com/nextcloud/documentation/actions/runs/7005702281/job/19055942324

[nickvergessen]

/backport to stable27

[provokateurin]

This doesn't make much sense because it breaks silently and not due to PR changes. The only real solution is #11318

[nickvergessen]

This doesn't make much sense because it breaks silently and not due to PR changes.

It would still notify us that the next release build will be broken.
But yeah #11318 is much better.

[provokateurin]

It would still notify us that the next release build will be broken.

In that case running it as a cron would make more sense since none of the PRs will be the cause of the breakage and it would unnecessarly block merging fine PRs.

[nickvergessen]

In that case running it as a cron would make more sense since none of the PRs will be the cause of the breakage and it would unnecessarly block merging fine PRs.

But no one would notice until a release, because no one is cron subscribed. Fixing is easy and straight forward once reported, so for now (until #11318 is there), I think this is a helpful step.