ENH: Implement new breathelinkstruct Directive

The original breathelink can only handle doxygenclass. Defining breathelinkstruct specifies this is a struct and will be processed as a doxygenstruct in the breathelink source code.

In addition, the documenation for how to write a new example has been
updated with the new directive.

Author: mseng10

Documentation/Contribute/WriteANewExample.rst

@@ -182,14 +182,15 @@ Classes demonstrated
 --------------------
 
 At the end of the example, provide quick reference and a link to the doxygen
-documentation for the important classes used in the example.  To do this, use
-the ``breathelink`` directive, as follows::
+documentation for the important classes or structs used in the example.  To do this, use
+the ``breathelink`` directive for C++ classes or ``breathelinkstruct`` directive for C++ structs, as follows::
 
   .. breathelink:: itk::BinaryDilateImageFilter
+  .. breathelinkstruct:: itk::Index
 
-Note that the breathelink directive is a custom one which makes use of:
+Note that ``breathelink`` and ``breathelinkstruct`` are custom directives which make the use of:
 
-- the `doxygenclass directive`_ provided by Breathe_ with the *:no-link:*
+- the `doxygenclass directive`_ or `doxygenstruct directive`_ provided by Breathe_ with the *:no-link:*
   option.
 - the `doxylink`_ directive which provides a link to the full doxygen
   documentation with an `external hyperlink target`_. Note that Doxygen URL's
@@ -198,6 +199,7 @@ Note that the breathelink directive is a custom one which makes use of:
 .. _Breathe:                   https://github.com/michaeljones/breathe
 .. _external hyperlink target: http://docutils.sourceforge.net/docs/user/rst/quickref.html#external-hyperlink-targets
 .. _doxygenclass directive:    https://breathe.readthedocs.io/en/latest/class.html
+.. _doxygenstruct directive:   https://breathe.readthedocs.io/en/latest/struct.html
 .. _figure directive:          http://docutils.sourceforge.net/docs/ref/rst/directives.html#figure
 .. _literalinclude directive:  https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-literalinclude
 .. _literal block:             http://docutils.sourceforge.net/docs/user/rst/quickref.html#literal-blocks

Utilities/SphinxExtensions/breathelink

@@ -45,4 +45,4 @@ Python
 Classes demonstrated
 --------------------
 
-.. breathelink:: itk::Offset itk::Index
+.. breathelinkstruct:: itk::Offset itk::Index

src/Core/Common/AddOffsetToIndex/Documentation.rst

@@ -39,4 +39,4 @@ Python
 Classes demonstrated
 --------------------
 
-.. breathelink:: itk::Index
+.. breathelinkstruct:: itk::Index

src/Core/Common/CreateAIndex/Documentation.rst

@@ -39,4 +39,4 @@ Python
 Classes demonstrated
 --------------------
 
-.. breathelink:: itk::Size
+.. breathelinkstruct:: itk::Size

src/Core/Common/CreateASize/Documentation.rst

@@ -40,4 +40,4 @@ Classes demonstrated
 --------------------
 
 .. breathelink:: itk::Point
-.. breathelink:: itk::Index
+.. breathelinkstruct:: itk::Index

src/Core/Common/DistanceBetweenIndices/Documentation.rst

@@ -44,4 +44,4 @@ Python
 Classes demonstrated
 --------------------
 
-.. breathelink:: itk::Size
+.. breathelinkstruct:: itk::Size