Docs and issue templates: clarify usage of issues and discussions (#5527)

This PR adds guidance and best practices on how to use issues and
discussions.
This is useful to avoid issues that should be discussions, for example.

---------

Co-authored-by: Edoardo Zoni <59625522+EZoni@users.noreply.github.com>
Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Author: 3 people

.github/ISSUE_TEMPLATE/blank_issue.md

@@ -0,0 +1,14 @@
+---
+name: Blank issue
+about: Ask us a question
+labels: [question]
+---
+
+Are you here because you have something to report that is neither a bug, a new feature, nor an installation problem?
+
+Before opening this issue, consider opening a [discussion](https://github.com/ECP-WarpX/WarpX/discussions) instead!
+
+Issues are used to report bugs, installation problems or to request new features.
+Discussions are used to ask more open-ended questions, brainstorm, ask our feedback, etc.
+
+You can find more details on how to use issues and discussions [here](https://github.com/ECP-WarpX/WarpX/blob/development/CONTRIBUTING.rst).

.github/ISSUE_TEMPLATE/config.yml

@@ -1 +1 @@
-blank_issues_enabled: true
+blank_issues_enabled: false

CONTRIBUTING.rst

@@ -12,6 +12,36 @@ Git workflow
 The WarpX project uses `git <https://git-scm.com>`_ for version control.
 If you are new to git, you can follow `this tutorial <https://swcarpentry.github.io/git-novice/>`__.
 
+
+What to do when
+^^^^^^^^^^^^^^^
+
+Issues
+""""""
+
+`Issues <https://github.com/ECP-WarpX/WarpX/issues>`__ are used to track tasks that the contributors and/or maintainers can work on.
+Use issues for reporting bugs or installation problems and for requesting new features.
+
+If you've found a bug and wish to report it, first search the open issues and `pull requests <https://github.com/ECP-WarpX/WarpX/pulls>`__ to see if someone else has already reported the same thing.
+If it's something new, open an issue using a template.
+We'll use the issue to address the problem you've encountered.
+
+Discussions
+"""""""""""
+
+`Discussions <https://github.com/ECP-WarpX/WarpX/discussions>`__ are for open-ended conversations, general questions, brainstorming ideas.
+Please, use discussions if you want to ask us something that is not technically a bug or a feature.
+Feel free to ping us there!
+
+Pull Requests (PRs)
+"""""""""""""""""""
+
+Open a `pull request <https://github.com/ECP-WarpX/WarpX/pulls>`__ if you want to add a new feature yourself.
+Follow the guide below for more details.
+
+
+Thank you for contributing! 🥰
+
 Configure your GitHub Account & Development Machine
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^