ARROW-15721: [Website] Add Flight/Flight SQL to subprojects

Closes #197 from lidavidm/arrow-15721:

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: lidavidm

_includes/header.html

@@ -55,6 +55,8 @@
              Subprojects
           </a>
           <div class="dropdown-menu" aria-labelledby="navbarDropdownSubprojects">
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/format/Flight.html">Arrow Flight</a>
+            <a class="dropdown-item" href="{{ site.baseurl }}/docs/dev/format/FlightSql.html">Arrow Flight SQL</a>
             <a class="dropdown-item" href="{{ site.baseurl }}/datafusion">DataFusion</a>
           </div>
         </li>