Add workflow to perform e2e tests against staging environment

Increasing Access

Unsure

Feature enhancement details

Following work by @Geethegreat for his GSOC project, there is now an e2e test suite that runs on a checked-out copy of the codebase, upon maintainer approval, for PR's pointing towards develop

Current during the weekly release process, there is a step to 'manual regression check on the staging site', but this can be automated by running e2e test suite against staging instead.

We should:

1. Refactor the e2e test suite so that the running of the test suite is a re-usable action
2. Add a new workflow that has:
   - has some setup steps to configure variables to enable the playwright bot to test on the staging site
   - should include some way to ensure that test runs do not collide with one another if one is triggered before a previous one is completed
   - ensure that secrets to access staging remain invisible
   - triggers the test suite with the re-usable action from step 1
   - has a manual trigger & is only able to be triggered by maintainers
3. Add a link to the new workflow in the release docs

Start by locating the existing e2e workflow and the release documentation referenced in the issue. Read how the current suite runs against a checked-out codebase, then trace the staging configuration and secret handling before separating reusable test execution from workflow setup. Done means a maintainer-triggered staging workflow runs safely without overlapping runs, keeps secrets hidden, and is linked from the release docs.