Utilities/Sphinx: Add option to build 'singlehtml' format

Add SPHINX_SINGLEHTML to enable the Sphinx 'singlehtml' builder.
Merge branch 'master' into doc-singlehtml

Resolve conflicts in Utilities/Sphinx/CMakeLists.txt by adding the help
options from both sides.
Merge topic 'doc-singlehtml' into next

aaa6c8a6 Merge branch 'master' into doc-singlehtml
faf291a9 Utilities/Sphinx: Add option to build 'singlehtml' format