[docs] Improve landing page (#12453)

This commit is intended to improve new user's first-interaction with the Sphinx site:

- Make page header icon/text smaller and not capitalized
- Give min-width to left sidebar (it was getting too small at certain window sizes)
- Replace features list on landing page with admonition boxes, with adaptive layout
- Add landing page "used by" section
- Slightly restructure the Extension section, into Tutorials and How-tos
- Add code to `conf.py` to write HTML write redirect pages for moved documents
- Improve support page, by adding link to Stackoverflow, GH discussion and ReadtheDocs,
  and remove defunct link to libera chat and matplotlib tutorial

doc/development/tutorials/helloworld.rst

@@ -5,7 +5,7 @@ The objective of this tutorial is to create a very basic extension that adds a
 new directive. This directive will output a paragraph containing "hello world".
 
 Only basic information is provided in this tutorial. For more information, refer
-to the :doc:`other tutorials <index>` that go into more details.
+to the :ref:`other tutorials <extension-tutorials-index>` that go into more details.
 
 .. warning::
 

doc/development/tutorials/index.rst

@@ -1,17 +1,12 @@
 .. _extension-tutorials-index:
 
-Extension tutorials
-===================
-
-Refer to the following tutorials to get started with extension development.
-
+Tutorials
+=========
 
 .. toctree::
-   :caption: Directive tutorials
-   :maxdepth: 1
+   :maxdepth: 2
 
    helloworld
    todo
    recipe
    autodoc_ext
-