Merge topic 'doc_ep'

c949a8742e ExternalProject: Add option subsection anchors for accessibility from TOC
6828b9df5a Help/dev: Extend documentation guide with another section nesting level

Acked-by: Kitware Robot <[email protected]>
Merge-request: !8657

Help/dev/documentation.rst

@@ -431,6 +431,7 @@ The section header underline character hierarchy is
 * ``-``: Subsection or `CMake Domain`_ object document title
 * ``^``: Subsubsection or `CMake Domain`_ object document section
 * ``"``: Paragraph or `CMake Domain`_ object document subsection
+* ``~``: `CMake Domain`_ object document subsubsection
 
 Style: Whitespace
 ^^^^^^^^^^^^^^^^^