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.

Modules/FetchContent.cmake

@@ -11,6 +11,12 @@ FetchContent
 
   .. contents::
 
+.. note:: The :guide:`Using Dependencies Guide` provides a high-level
+  introduction to this general topic. It provides a broader overview of
+  where the ``FetchContent`` module fits into the bigger picture,
+  including its relationship to the :command:`find_package` command.
+  The guide is recommended pre-reading before moving on to the details below.
+
 Overview
 ^^^^^^^^
 
@@ -720,7 +726,7 @@ frameworks are available to the main build:
   # Catch2 will be available to the rest of the build
   FetchContent_MakeAvailable(googletest Catch2)
 
-.. _FetchContent-find_package-integration:
+.. _FetchContent-find_package-integration-examples:
 
 Integrating With find_package()
 """""""""""""""""""""""""""""""