Documentation workflow does not run for pull requests targeting develop

[thc1006]

Describe the bug

The Documentation workflow only runs for master:

on:
  pull_request:
    types: [opened, synchronize, reopened, ready_for_review]
    branches: [master]
    paths:
      - "docs/**"
      - "rocketpy/**"
      ...
  push:
    branches: [master]

Contributions do not go to master. Of the last 30 pull requests, 25 target develop and 3 target master. So the build that runs with -W --keep-going sees almost nothing before it is merged, and a docs change reaches it only once develop is promoted, in a batch, away from the change that caused it.

The path filter already limits this to work that can affect the docs, and rocketpy/** is in that filter, so the gap is not only about docs/. A docstring edit in a PR to develop can break the autodoc reference and nothing says so.

To Reproduce

#1097 changes docs/user/custom_sampler.rst and targets develop. Its only check is the auto-assign workflow. No documentation build ran.

Expected behavior

Add develop to both branch filters:

    branches: [master, develop]

The job already sets DOCS_SKIP_EXECUTE: "1", so it renders jupyter-execute cells as static blocks and makes no network calls. The last six runs took between 2m18s and 2m31s, and the path filter keeps it off PRs that cannot affect the docs.

If running it on every develop PR is more than you want, the pull_request filter alone would still close most of the gap, since that is where the feedback is useful.

Additional context

Found while opening #1097, where the reStructuredText I added had nothing behind it but a local sphinx-build. I would rather this were checked by CI than by whether I remembered to run it.

Happy to send the one-line PR if you want it.

Verified against develop at 1691119.

Signed-off-by: thc1006 84045975+thc1006@users.noreply.github.com