Help: Overhaul and expand the Using Dependencies Guide

The guide previously only focused on the find_package() command,
with a bias towards libraries. FetchContent was not mentioned at all.
Reorganise and update the existing content. Add new sections to cover
providing dependencies with FetchContent and dependency providers.
Improve discoverability of the guide by mentioning it at the beginning
of the find_package(), FetchContent and dependency provider docs.

Help/command/cmake_language.rst

@@ -236,6 +236,9 @@ Dependency Providers
 
 .. versionadded:: 3.24
 
+.. note:: A high-level introduction to this feature can be found in the
+          :ref:`Using Dependencies Guide <dependency_providers_overview>`.
+
 .. code-block:: cmake
 
   cmake_language(SET_DEPENDENCY_PROVIDER <command>

Help/command/find_package.rst

@@ -11,6 +11,12 @@ find_package
 
    .. contents::
 
+.. note:: The :guide:`Using Dependencies Guide` provides a high-level
+  introduction to this general topic. It provides a broader overview of
+  where the ``find_package()`` command fits into the bigger picture,
+  including its relationship to the :module:`FetchContent` module.
+  The guide is recommended pre-reading before moving on to the details below.
+
 Find a package (usually provided by something external to the project),
 and load its package-specific details.  Calls to this command can also
 be intercepted by :ref:`dependency providers <dependency_providers>`.