Create a "Testing Applications" subsection in devel-examples

Live previews: 
http://bitcoindevtest.us.to/en/developer-examples
http://bitcoindevtest.us.to/en/developer-reference
http://bitcoindevtest.us.to/en/developer-guide

This pull request mostly re-uses existing (great) regtest / testnet texts and moves them into a dedicated subsection for higher visibility and so anyone can easily refer & link to them.

Since they are more likely to be referred often from devel-examples and not from devel-guide, I've also moved the "Bitcoin Core setup instructions", and applied a consistent introduction for each page (this also lets us avoid repeating some of this information in "Transaction" examples).

I also needed to fix one regex and some code blocks syntax to prevent autocrossref.rb from inserting links in code blocks (it was replacing "regest" in some of them).

@harding Agreed on re-accessing where this should be located later. I was (maybe wrongly) assuming regtest / testnet will be used often (not just from the Transaction examples). Anyway I think the important part is to have these instructions easily "findable" and "linkable". I'm not sure about having the same "Bitcoin Core" subsection in devel-[guide/ref/examples] though, as it seemed good to me to avoid duplicating content, and mixing examples / references / guide content altogether.