Add documentation and comment to GH workflow (template)

[ckunki]

Ticket #771 contains a comment regarding the different cases in which report should be called
File report.yml is called and in which file metric.json should be generated.

As this is not obvious, the current ticket requests adding a short explanation to the User guide and linking it from the workflows, in particular from merge-gate.yml:

  approve-run-slow-tests:
    name: Approve Running Slow Tests?
    if: ${{ inputs.root-event != 'schedule' }}

Rationale, see https://github.com/exasol/python-toolbox/pull/770/changes#r3192157588

if special handling is necessary in a workflow, a comment in the workflow is strongly recommended
because the workflows are use in many projects and create the connection from there to the explanation in a ticket is almost impossible.
Otherwise users are forced to confirm, the change coming from the PTB,
then look in the PTB for the workflow and then do Blame to find which commit introduced it
and then actually look for scheduled run to see how it behaves.
If the explanation is too long, put it somewhere else and link to it.

Changes

• User Guide is updated to explain under which conditions
  • manual approval is required
  • metrics should be generated
  • report should be called
• link to the section in the user guide is added to all relevant places in the GH workflows
• GH workflow templates are updated, too