Skip docs preview for dependabot PRs

[lgoettgens]

Unlike workflows triggered by other actors, this means [workflows triggered by dependabot] receive a read-only GITHUB_TOKEN and do not have access to any secrets that are normally available. This will cause any workflows that attempt to write to the repository to fail when they are triggered by Dependabot.
(https://docs.github.com/en/code-security/dependabot/troubleshooting-dependabot/troubleshooting-dependabot-on-github-actions#troubleshooting-failures-when-dependabot-triggers-existing-workflows)

This means that pushing the documentation preview will fail for dependabot PRs (see e.g. https://github.com/oscar-system/Oscar.jl/actions/runs/13183920804/job/36809531604?pr=4550#step:6:622). Thus, we should skip the preview in these cases.

[thofma]

does every repo need this?

[lgoettgens]

does every repo need this?

Only the ones that have doc previews enabled.
Let's first try if it indeed works here, before spending more energy on the other repos.

[fingolfin]

If this works, it works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.43%. Comparing base (069bd41) to head (0571c38).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4554      +/-   ##
==========================================
+ Coverage   84.41%   84.43%   +0.02%     
==========================================
  Files         672      673       +1     
  Lines       89155    89624     +469     
==========================================
+ Hits        75256    75673     +417     
- Misses      13899    13951      +52     

see 11 files with indirect coverage changes