Improve CI for documentation

For the CI we can improvise some points in the doc generation:

- [ ] - revisit the build folder for the generated docs, see https://github.com/eclipse-iceoryx/iceoryx2/pull/990#discussion_r2331216754
- [ ] - revisit the nightly job for documentation in `.github/workflows/documentation.yml`
- [ ] - check if we can remove some logic of `.github/workflows/documentation.yml` into separate scripts

Start by reading the discussion on PR #990 and inspect the generated documentation build folder. Then review .github/workflows/documentation.yml, including its nightly job and embedded logic. Done means the documentation CI is simplified, with any suitable workflow logic moved into separate scripts and generation still working.

Written by the indexing model from the issue text.