Help/dev: Document use of sphinx-copybutton

Add a section on third-party Sphinx extensions. While we're here, add
a cross-reference on CMake's `README.rst`.

Help/dev/documentation.rst

@@ -24,6 +24,24 @@ repository to ``build/html`` and ``build/man`` directories:
   $ cmake -S Utilities/Sphinx -B build -DSPHINX_HTML=ON -DSPHINX_MAN=ON
   $ cmake --build build
 
+Depending on how Sphinx is installed on the system, optionally configure with
+``-DSPHINX_EXECUTABLE=/path/to/sphinx-build``.
+
+Sphinx Extensions
+-----------------
+
+When building the HTML help, CMake uses the following third-party
+`extensions`_:
+
+* `sphinx-copybutton`_: Adds an interactive copy button to the corner of
+  ``code-block`` directives. To generate the documentation locally with this
+  extension, configure CMake as above with ``-DSPHINX_HTML_COPYBUTTON=ON``.
+  Ensure the extension is installed in the same environment (or on the system)
+  as the ``SPHINX_EXECUTABLE``.
+
+.. _`extensions`: https://www.sphinx-doc.org/en/master/usage/extensions/index.html
+.. _`sphinx-copybutton`: https://sphinx-copybutton.readthedocs.io/en/latest/
+
 Markup Constructs
 -----------------
 

README.rst

@@ -57,12 +57,14 @@ generator and options.  Then build it and install it.
 To build the documentation, install `Sphinx`_ and configure CMake with
 ``-DSPHINX_HTML=ON`` and/or ``-DSPHINX_MAN=ON`` to enable the "html" or
 "man" builder.  Add ``-DSPHINX_EXECUTABLE=/path/to/sphinx-build`` if the
-tool is not found automatically.
+tool is not found automatically.  See the `CMake Documentation Guide`_ for
+details.
 
 To run the test suite, run ``ctest`` in the CMake build directory after
 building.  See the `CMake Testing Guide`_ for details.
 
 .. _`Sphinx`: https://sphinx-doc.org
+.. _`CMake Documentation Guide`: Help/dev/documentation.rst
 .. _`CMake Testing Guide`: Help/dev/testing.rst
 
 Building CMake from Scratch