Adapt the PR template to make the possible header "tags" available

We have a bunch of "tags" (sorry if the name is not fully correct) like:
`docs-only`, `tests-only`, `full-ci` etc

which can be put as leading part in the headline. Such a headline would then look like:

`[docs-only] Some description changes for service xyz.`

It would be great if the PR tempate could announce these templates that can be used and gives a brief description what the impact is.

This would help non-hardcore-dev stuff to select the correct "tag" to optimize CI ressources.

@micbar @butonic @kobergj