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).

"Explanations" sounds worse to my ear than the original "information", possibly because information is neutral but explanation is often used in modern English to mean "justification of wrongdoing."