Create a "Testing Applications" subsection in devel-examples

Move existing "regtest / testnet" texts to the new subsection and link to it
Move Bitcoin Core setup instructions in devel-examples
Add a consistent introduction for devel-(guide/ref/examples)
Fix autocrossref.rb to not add links inside {% highlight %} code blocks

_includes/example_testing.md

@@ -0,0 +1,91 @@
+## Testing Applications
+
+{% autocrossref %}
+
+Bitcoin Core provides testing tools designed to let developers
+test their applications with reduced risks and limitations.
+
+{% endautocrossref %}
+
+### Testnet
+
+{% autocrossref %}
+
+When run with no arguments, all Bitcoin Core programs default to Bitcoin's main
+network ([mainnet][mainnet]{:#term-mainnet}{:.term}). However, for development,
+it's safer and cheaper to use Bitcoin's test network (testnet)
+where the satoshis spent have no real-world value. Testnet also relaxes some
+restrictions (such as standard transaction checks) so you can test functions
+which might currently be disabled by default on mainnet.
+
+To use testnet, use the argument <!--[-->`-testnet`<!--]--> with `bitcoin-cli`, `bitcoind` or `bitcoin-qt` or add
+<!--[-->`testnet=1`<!--]--> to your `bitcoin.conf` file.  To get
+free satoshis for testing, use [Piotr Piasecki's testnet faucet][].
+Testnet is a public resource provided for free by members of the
+community, so please don't abuse it.
+
+{% endautocrossref %}
+
+### Regtest Mode
+
+{% autocrossref %}
+
+For situations
+where interaction with random peers and blocks is unnecessary or
+unwanted, Bitcoin Core's regression test mode (regtest mode) lets you
+instantly create a brand-new private block chain with the same basic
+rules as testnet---but one major difference: you choose when to create
+new blocks, so you have complete control over the environment.
+
+Many developers consider regtest mode the preferred way to develop new
+applications. The following example will let you create a regtest
+environment.
+
+{% endautocrossref %}
+
+{% highlight bash %}
+> bitcoind -regtest -daemon
+Bitcoin server starting
+{% endhighlight %}
+
+{% autocrossref %}
+
+Start `bitcoind` in regtest mode to create a private block chain.
+
+{% endautocrossref %}
+
+~~~
+bitcoin-cli -regtest setgenerate true 101
+~~~
+
+{% autocrossref %}
+
+Generate 101 blocks using a special version of the `setgenerate` RPC
+which is only available in regtest mode. This takes about 30 seconds on
+a generic PC. Because this is a new block chain using Bitcoin's default
+rules, the first 210,000 blocks pay a block reward of 50 bitcoins.
+However, a block must have 100 confirmations before that reward can be
+spent, so we generate 101 blocks to get access to the coinbase
+transaction from block #1.
+
+{% endautocrossref %}
+
+{% highlight bash %}
+bitcoin-cli -regtest getbalance
+50.00000000
+{% endhighlight %}
+
+{% autocrossref %}
+
+Verify that we now have 50 bitcoins available to spend.
+
+You can now use Bitcoin Core RPCs prefixed with <!--[-->`bitcoin-cli -regtest`<!--]-->.
+
+Regtest wallets and block chain state (chainstate) are saved in the <!--[-->`regtest`<!--]-->
+subdirectory of the Bitcoin Core configuration directory. You can safely
+delete the <!--[-->`regtest`<!--]--> subdirectory and restart Bitcoin Core to
+start a new regtest. (See the [Developer Examples Introduction][devexamples] for default
+configuration directory locations on various operating systems. Always back up
+mainnet wallets before performing dangerous operations such as deleting.)
+
+{% endautocrossref %}