Fix the `WriteANewExample` documentation section references

The section references in the [WriteANewExample.rst](https://github.com/InsightSoftwareConsortium/ITKExamples/blob/master/Documentation/Contribute/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 ` 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`