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 think it isn't that important---one of the things I like about the way Kramdown creates its automatic head anchors is that they're tied to the text, not the header depth.  That means we can always rearrange sections and subsections later.  I say we try it your way and see how we feel about it in a few months.