Replace documentation URL from RDoc-generated content to RubyDoc.info (https://www.rubydoc.info/gems/snmp/)

Author: taketo1113

docs/index.html

@@ -106,13 +106,13 @@ <h3>Import MIBs</h3>
 
     <p>If you don't provide a MIB list then the default MIBs are loaded: SNMPv2-MIB, IF-MIB, IP-MIB, TCP-MIB, UDP-MIB</p>
 
-    <p>If you want to import custom MIBs, then have a look at the <a href="doc/index.html">documentation</a> for the <code>SNMP::MIB</code> class.  If you use the <code>import_module</code> method, you will need to install <a href="https://www.ibr.cs.tu-bs.de/projects/libsmi/">libsmi</a> first, or create the required YAML file manually.</p>
+    <p>If you want to import custom MIBs, then have a look at the <a href="https://www.rubydoc.info/gems/snmp/">documentation</a> for the <code>SNMP::MIB</code> class.  If you use the <code>import_module</code> method, you will need to install <a href="https://www.ibr.cs.tu-bs.de/projects/libsmi/">libsmi</a> first, or create the required YAML file manually.</p>
 
     <h3>Start Coding</h3>
 
-    <p>Have a look at the <a href="doc/index.html">documentation</a> for the <code>SNMP::Manager</code> class.  This class provides the main interface for sending and receiving SNMP PDUs.</p>
+    <p>Have a look at the <a href="https://www.rubydoc.info/gems/snmp/">documentation</a> for the <code>SNMP::Manager</code> class.  This class provides the main interface for sending and receiving SNMP PDUs.</p>
 
-<p>Here's an example to get you started.  You can find plenty more in the <a href="doc/index.html">README</a> file</p>    
+<p>Here's an example to get you started.  You can find plenty more in the <a href="https://www.rubydoc.info/gems/snmp/">README</a> file</p>
 <pre><code>
 require 'snmp'
 
@@ -136,7 +136,7 @@ <h3>Net-SNMP</h3>
 
 	</div>
 	<div id="sidebar">
-	  <h3><a href="doc/index.html">Documentation &raquo;</a></h3>
+	  <h3><a href="https://www.rubydoc.info/gems/snmp/">Documentation &raquo;</a></h3>
     <h3><a href="https://rubygems.org/gems/snmp/">Download &raquo;</a></h3>
     <h3>Happy Users</h3>
     <div id="quotes">