ci(docs): Fix publish job

[g-braeunlich]

Description

Splitting the docs job into build and publish caused a new problem:
The _build folder does not propagate from the build to the publish job.
As a fix, an artifact is uploaded in the build job and downloaded in the publish job.

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files
• I have run ruff check . and ruff format
• I have run mypy .
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[g-braeunlich]

@ffelten This one should really work now. Tested it by pushing a temporary commit which also tested the deploy step (temp commit removed again).