docs: move Manpages to top-level of documentation hierarchy

refs #2730

Change-Id: I0b1d020b5b856498a50158d70f79ae6d9174e58c
diff --git a/docs/index.rst b/docs/index.rst
index 9182cc1..5543840 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -18,6 +18,7 @@
    INSTALL
    examples
    tutorials
+   manpages
    code-style
 
 - :doc:`README`
@@ -31,7 +32,9 @@
    + :doc:`tutorials/security-library`
    + :doc:`tutorials/utils-ndn-regex`
    + :doc:`tutorials/security-validator-config`
-   + :doc:`manpages`
+   + :doc:`tutorials/signed-interest`
+
+- :doc:`manpages`
 
 Documentation for ndn-cxx developers and contributors
 +++++++++++++++++++++++++++++++++++++++++++++++++++++