build: align minimum build dependencies with ndn-cxx

 * Recommend boost >= 1.65.1 and gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7
 * Fix redundant-tags warning with gcc 10
 * Add CentOS-specific instructions to docs/INSTALL.rst
 * Add Xcode 11.5 to Travis CI

Refs: #5087, #5106
Change-Id: I70746729a132ecb07d4788934385bb403039dba9
diff --git a/docs/named_data_theme/layout.html b/docs/named_data_theme/layout.html
index 937f8e2..c9fb5c1 100644
--- a/docs/named_data_theme/layout.html
+++ b/docs/named_data_theme/layout.html
@@ -38,7 +38,7 @@
           <h3>{{ _('Table Of Contents') }}</h3>
           {{ toctree(includehidden=True) }}
 
-          <h3>{{ _('Additional documenation') }}</h3>
+          <h3>{{ _('Additional documentation') }}</h3>
           <ul>
             <li class="toctree-l1"><a class="reference external" href="https://redmine.named-data.net/projects/nfd/wiki">NFD Wiki</a></li>
             <li class="toctree-l1"><a class="reference internal" href="doxygen/annotated.html">API documentation (doxygen)</a></li>