doc: Improve "Contributing to Zephyr" landing page

The "Contributing to Zephyr" page used to be a simple table of contents
with pointers to sub-pages. This bland page was not very inviting to new
contributors, and provided little to no context as to what to expect in
each sub-section. This commit improves the landing page by implenmenting
the following changes:

- Remove auto toctree and replace with manually curated entries that
  provide more context about each sub-section.
- Add a section about the Zephyr Contributor Badge.
- Add a section about getting help, with links to Discord and dev
  mailing list.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/contribute/documentation/index.rst

@@ -1,10 +0,0 @@
-.. _documentation:
-
-Documentation
-#############
-
-.. toctree::
-   :maxdepth: 1
-
-   guidelines.rst
-   generation.rst

doc/contribute/index.rst

@@ -3,13 +3,108 @@
 Contributing to Zephyr
 ######################
 
+Contributions from the community are the backbone of the project. Whether it is by submitting code,
+improving documentation, or proposing new features, your efforts are highly appreciated. This page
+lists useful resources and guidelines to help you in your contribution journey.
+
+General Guidelines
+==================
+
 .. toctree::
    :maxdepth: 1
+   :hidden:
 
    guidelines.rst
-   contributor_expectations.rst
-   proposals_and_rfcs.rst
    coding_guidelines/index.rst
-   documentation/index.rst
+   proposals_and_rfcs.rst
+   contributor_expectations.rst
+
+:ref:`contribute_guidelines`
+   Learn about the overall process and guidelines for contributing to the Zephyr project.
+
+   This page is a mandatory read for first-time contributors as it contains important information on
+   how to ensure your contribution can be considered for inclusion in the project and potentially
+   merged.
+
+:ref:`contributor-expectations`
+   This document is another mandatory read that describes the expected behavior of `all`
+   contributors to the project.
+
+:ref:`coding_guidelines`
+   Code contributions are expected to follow a set of coding guidelines to ensure consistency and
+   readability across the code base.
+
+   This page describes these guidelines and introduces important considerations regarding the use of
+   :ref:`inclusive language <coding_guideline_inclusive_language>`.
+
+:ref:`rfcs`
+   Learn when and how to submit RFCs (Request for Comments) for new features and changes to the
+   project.
+
+Documentation
+=============
+
+The Zephyr project thrives on good documentation. Whether it is as part of a code contribution or
+as a standalone effort, contributing documentation is particularly valuable to the project.
+
+.. toctree::
+   :maxdepth: 1
+   :hidden:
+
+   documentation/guidelines.rst
+   documentation/generation.rst
+
+:ref:`doc_guidelines`
+   This page provides some simple guidelines for writing documentation using the reSTructuredText
+   (reST) markup language and Sphinx documentation generator.
+
+:ref:`zephyr_doc`
+   As you write documentation, it can be helpful to see how it will look when rendered.
+
+   This page describes how to build the Zephyr documentation locally.
+
+
+Dealing with external components
+================================
+
+.. toctree::
+   :maxdepth: 1
+   :hidden:
+
    external.rst
    bin_blobs.rst
+
+:ref:`external-contributions`
+   Basic functionality or features that would make useful addition to Zephyr might be readily
+   available in other open source projects, and it is recommended and encouraged to reuse such code.
+   This page describes in more details when and how to import external source code into Zephyr.
+
+:ref:`external-tooling`
+   Similarly, external tooling used during compilation, code analysis, testing or simulation, can be
+   beneficial and is covered in this section.
+
+:ref:`bin-blobs`
+   As some functionality might only be made available with the help of executable code distributed
+   in binary form, this page describes the process and guidelines for :ref:`contributing binary
+   blobs <blobs-process>` to the project.
+
+Zephyr Contributor Badge
+========================
+
+When your first contribution to the Zephyr project gets merged, you'll become eligible to claim your
+Zephyr Contributor Badge. This digital badge can be displayed on your website, blog, social media
+profile, etc. It will allow you to showcase your involvement in the Zephyr project and help raise
+its awareness.
+
+You may apply for your Contributor Badge by filling out the `Zephyr Contributor Badge form`_.
+
+Need help along the way?
+========================
+
+If you have questions related to the contribution process, the Zephyr community is here to help.
+You may join our Discord_ channel or use the `Developer Mailing List`_.
+
+
+.. _Discord: https://chat.zephyrproject.org
+.. _Developer Mailing List: https://lists.zephyrproject.org/g/devel
+.. _Zephyr Contributor Badge form: https://forms.gle/oCw9iAPLhUsHTapc8