Add workflow to perform e2e tests against staging environment

[clairep94]

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

[clairep94]

cc: @khanniie @raclim @doradocodes @Geethegreat