docs(developing): adapt documentation after infrastructure code split

Adapts documentation for data curators following up the split of the
portal's infrastructure code from the content in two repositories.

Implements comments from the #3590 pull request review.

Upgrades the `cernopendata-portal` image version to 0.2.9.

Closes #3678.

CONTRIBUTING.rst

@@ -19,33 +19,26 @@ portal developments, you can `watch ongoing discussions
 `become part of the team
 <https://github.com/orgs/cernopendata/teams>`_.
 
-Code contributions
-------------------
+Contributions
+-------------
 
 We follow typical `GitHub flow
-<https://guides.github.com/introduction/flow/index.html>`_.
+<https://docs.github.com/en/get-started/using-github/github-flow>`_.
 
 1. Fork this repository into your personal space.
-2. Start a new topical branch for any contribution.  Name it sensibly,
-   say ``fix-event-display-icons``.
-3. Test your branch on a local site.  If everything works as expected,
-   please `sign your commits
-   <https://invenio.readthedocs.io/en/latest/technology/git.html#r2-remarks-on-commit-log-messages>`_
-   to indicate its quality.
-4. Create `logically separate commits for logically separate things
-   <https://invenio.readthedocs.io/en/latest/technology/git.html#r1-remarks-on-commit-history>`_.
-   Check out our usual `development practices
-   <https://invenio-github.readthedocs.io/en/latest/contributing.html>`_.
-5. Please add any ``(closes #123)`` directives in your commit log
-   message if your pull request closes an open issue.
-6. Issue a pull request.  If the branch is not quite ready yet, please
-   indicate ``WIP`` (=work in progress) in the pull request title.
-
-For more information on how we work with branches, see our `developing
-guide <DEVELOPING.rst>`_.
-
-Chatroom
---------
-
-Our chatroom is `on gitter
-<https://gitter.im/cernopendata/opendata.cern.ch>`_.
+2. Start a new topical branch for any contribution. Name it descriptively, for
+   example ``fix-cms-2012-collision-energy``.
+3. Ideally, test your branch on a local development site to see if everything
+   works correctly.
+4. Open a pull request against this repository. Verify that all the continuous
+   integration checks are passing. If not, please amend the pull request
+   accordingly.
+
+For more information on the process, please see our `developing guide
+<DEVELOPING.rst>`_.
+
+Support
+-------
+
+You can also get in touch via our `Mattermost
+<https://mattermost.web.cern.ch/it-dep/channels/opendata>`_ chat room.