This commit merges the RPC introduction section (h4), "JSON-RPC &
bitcoin-cli" into the currently-empty RPC section (h3). To do this, it
splits the contents of the file `_includes/ref_core_rpc_intro.md` into
the new files `_includes/ref/bitcoin-core/api-intro.md` and
`_includes/ref/bitcoin-core/rpcs/intro.md`
