build: align minimum build dependencies with ndn-cxx

 * Recommend gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7
 * Make graphviz optional for building documentation
 * Sync sphinx configuration

Change-Id: Iff1920b0b5580d58db9aeef11298f11e22d9b3cb
diff --git a/docs/index.rst b/docs/index.rst
index de90ecc..4bfa906 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -18,10 +18,10 @@
    ndns-db-manage
    manpages
 
-**Additional documentation**
+Additional Documentation
+^^^^^^^^^^^^^^^^^^^^^^^^
 
 * `API documentation (doxygen) <doxygen/annotated.html>`_
-
 * `Addressing Operational Challenges in Named Data Networking Through NDNS Distributed
   Database <http://lasr.cs.ucla.edu/afanasyev/data/files/Afanasyev/afanasyev-phd-thesis.pdf>`_
   (PhD Thesis, outlining purpose and overall design of the NDNS system)
@@ -29,7 +29,7 @@
 Downloading
 -----------
 
-* `Source code GitHub git repository <https://github.com/named-data/NDNS>`_.
+* `Source code git repository <https://github.com/named-data/NDNS>`_
 
 License
 -------