[docs 1/5] Port existing HTML manual to doxygen

* The existing HTML documentation under doc/html-manual has been
  converted to Markdown. This was done automatically using Pandoc, plus
  some manual work to give identifiers to sections and changing internal
  links to point to those sections with \ref.

* The Doxygen front page now contains some content: a link to the
  doxygen-ated HTML manual, and a note about the API documentation. The
  intention here is that the entire Doxygen site could be hosted
  publicly, serving both users of and contributors to CBMC from a single
  site.

* The doxyfile is updated to enable these changes.

Author: karkhaz

doc/CPPLINT.cfg

@@ -0,0 +1 @@
+exclude_files=.*

doc/architectural/front-page.md

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+CProver Documentation
+=====================
+
+These pages contain both user tutorials and automatically-generated API
+documentation. Users can download CProver tools from the
+<a href="http://www.cprover.org/">CProver website</a>; contributors
+should use the <a href="https://github.com/diffblue/cbmc">repository</a>
+hosted on GitHub.
+
+### For users:
+
+* The \ref cprover-manual "CProver Manual" details the capabilities of
+  CBMC and SATABS and describes how to install and use these tools. It
+  also covers the underlying theory and prerequisite concepts behind how
+  these tools work.
+
+### For contributors:
+
+* If you already know exactly what you're looking for, the API reference
+  is generated from the codebase. You can search for classes and class
+  members in the search bar at top-right or use one of the links in the
+  sidebar.
+
+\defgroup module_hidden _hidden