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).

I'm wondering if there's a way to have all three documents use the same h2 sections (or almost the same).  What do you think about changing this to an h3 under an h2 titled "Bitcoin Core", and then changing the Bitcoin Core APIs h2 in devref to also be "Bitcoin Core"?  (At some point we might even add a Bitcoin Core h2 to the devguide.)