ci: trigger docs quickstart tests on release tags

[JakeSCahill]

Summary

Proposal from the docs team: on v* release tags, also send a trigger-tests repository dispatch to redpanda-data/docs with the tag as console_version.

Why

The docs repo runs a Doc Detective quickstart test that drives the Console UI end to end (login, topics, filters, user creation, screenshots for the docs). When a Console release changes the UI, documented procedures and screenshots can silently break. The most recent example: a UI rework changed the topic detail tabs and the create-user dialog, which broke the docs test in a way that only surfaced through the docs repo's nightly run, and the published screenshots drifted out of date for months (fixed in redpanda-data/docs#1846).

With this dispatch, the docs repo tests the new Console version at release time and files an issue in the docs repo when documented flows or screenshots break. The receiving side already supports this: redpanda-data/docs#1848 maps the console_version payload to the test's version override.

Open question for the Console team (why this PR is a draft)

At tag-push time, is the release image for the new tag already published to docker.redpanda.com/redpandadata/console? If images publish later in the release pipeline (for example in console-enterprise), this dispatch would race image availability, and the right place for it is at the end of that pipeline instead. Happy to move it — the payload contract stays the same.

No new secrets: uses the same actions_bot_token and dispatch action as the existing steps in this workflow.

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