ci: adjust workflow target of documentation build

[johannaSommer]

Description

In line with our efforts to use the GitHub action resources mindfully, this PR proposes to run the documentation after a merge to main or on dispatch (with the click of a button), similar to the installation workflow.
Rationale:

• this workflow is only relevant for a PR adding a documentation snippet or a new algorithm. Since this is the case for a smaller-ish portion of the PRs I would propose this change - if needed this can be run by the PR opening person or reviewer.
• to catch any mistakes that might have been overlooked this is run once when a PR is merged to main. Even in the case where it fails, this is fine, as the documentation will only be built on a new release and we will be able to fix it until then.

Related Issue

None.

Type of Change

• 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

How Has This Been Tested?

None - the workflow target is tested already with the installation and works as intended.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• 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

Additional Notes

Quickly also added the permissions for the workflow in this PR - this is recommended by the GitHub security actions.

[sharpenb]

I like it!

[johnrachwan123]

LGTM