Create UG examples documentation from the respective MarkDown READMEs

[arporter]

At the moment we have to document the examples twice - once in doc/user_guide/examples.rst and again in the appropriate README.md under the examples directory. In #1669 @hiker suggested finding a way of removing this duplication.

I've googled and it looks as though https://github.com/CrossNox/m2r2 is the answer - it converts MD to RST and allows the results to be included in an existing RST file.

[arporter]

This would also be useful in the PSyAD documentation where the Examples section is sparse but there's a lot of content in the READMEs.