Fix the `WriteANewExample` documentation section references

[jhlegarreta]

The section references in the WriteANewExample.rst contributing documentation file seem not to be correctly built/do not point to the desired anchor and are not correctly rendered. e.g.

In this section we describe how to write an example by walking through the sections of an
 example's reStructuredText file. The mechanics of submitting an example via
 :ref:`Git/Gerrit directly <text-editor-submission>` are covered in other sections.

points to https://github.com/InsightSoftwareConsortium/ITKExamples/blob/master/Documentation/Contribute/WriteANewExample.rst#id1 or

For this reason, we recommend to use the provided python script CreateNewExample.py see 
:ref:`CreateNewExample`

points to https://github.com/InsightSoftwareConsortium/ITKExamples/blob/master/Documentation/Contribute/WriteANewExample.rst#id3