Contributions by @saivann to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad

en/developer-reference.md

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+---
+layout: base
+lang: en
+id: developer-reference
+title: "Developer Reference - Bitcoin"
+---
+
+# Bitcoin Developer Reference
+
+<p class="summary">Find technical specifications and API documentation.</p>
+
+<div markdown="1" id="toc" class="toc"><div markdown="1">
+
+* Table of contents
+{:toc}
+
+<ul><li><a href="/en/developer-documentation">Return To Overview</a></li></ul>
+
+</div></div>
+<div markdown="1" class="toccontent">
+
+{% include ref_block_chain.md %}
+{% include ref_transactions.md %}
+{% include ref_wallets.md %}
+{% include ref_payment_processing.md %}
+
+## Bitcoin Core APIs
+
+<!-- TODO, Relevant links:
+-- * https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_Calls_list
+-- * https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)
+-->
+
+### Remote Procedure Calls (RPCs)
+
+{% include ref_core_rpcs-abcdefg.md %}
+{% include ref_core_rpcs-hijklmn.md %}
+{% include ref_core_rpcs-opqrst.md %}
+{% include ref_core_rpcs-uvwxyz.md %}
+{% include references.md %}
+
+</div>
+
+<script>updateToc();</script>