[IMP] developer: improve navigation in top-level pages

Prior to this commit, users had to either know in advance or guess the
location of the content they were looking for. Top-level pages of the
"Developer" section of the documentation, in particular the "Developer"
page itself, were listing their sub-pages without directions for users.

This commit brings the following changes to improve the navigation:
- add directions for users on the "Developer" page and list the three
  main categories of developer documentation ("Tutorials", "How-to
  guides", and "Reference") with explanations of their content and
  target audience;
- add categories for content cards on the "Tutorials" and "How-to
  guides" pages, and fine-tune the toctree of the "Reference" page to
  more easily locate specific topics;
- clarify what are the "Python framework" and the "JavaScript framework"
  by relabelling them to "Server framework" and "Web framework" on
  top-level pages, as some users were confused to find that the JS
  framework was not responsible for the server, and others that the
  documentation for QWeb template is located in the Python documentation;
- extract the "Setup guide" from the "Getting started" tutorial and
  rename the latter to "Server framework 101" to allow reusing the setup
  guide in other tutorials and make clear that the "Server framework 101"
  tutorial is not about the Web framework.

task-3802536

closes odoo/documentation#8710

X-original-commit: 7f623b6ad5
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Co-authored-by: Valeriya (vchu) <vchu@odoo.com>

content/contributing.rst

@@ -48,7 +48,8 @@ lists the most important of them.
        pull requests from other contributors. By contributing to the codebase, you can make a direct
        and lasting impact on the quality and features of Odoo.
      - * :doc:`Guide: Contributing to the codebase <contributing/development>`
-       * :doc:`Developer tutorials </developer/howtos>`
+       * :doc:`Developer tutorials </developer/tutorials>`
+       * :doc:`How-to guides </developer/howtos>`
      - Developers with experience in Python and web development
    * - Contribute to the documentation
      - Submit corrections and improvements to the Odoo documentation. The product evolves fast, and