_content/doc: add redirect and fix links to /doc/install/source

A redirect for the old /doc/install-source.html page was missed
in CL 296383. Use .md suffix for consistency with similar pages.

Update some links to point to the new canonical URLs without
the .html extension to make them shorter and save a redirect.

Fixes golang/go#46545.

Change-Id: I117ceb42bb6f97b370808d0e27ff2eaaaa5a472e
Reviewed-on: https://go-review.googlesource.com/c/website/+/324771
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/doc/install-source.md

@@ -0,0 +1,3 @@
+<!--{
+	"Redirect": "/doc/install/source"
+}-->

_content/doc/install.html

@@ -7,11 +7,11 @@
 <p>For other content on installing, you might be interested in:</p>
 <ul>
   <li>
-    <a href="/doc/manage-install.html">Managing Go installations</a> -- How to
+    <a href="/doc/manage-install">Managing Go installations</a> -- How to
     install multiple versions and uninstall.
   </li>
   <li>
-    <a href="/doc/install-source.html">Installing Go from source</a> -- How to
+    <a href="/doc/install/source">Installing Go from source</a> -- How to
     check out the sources, build them on your own machine, and run them.
   </li>
 </ul>

_content/doc/install/gccgo.html

@@ -15,7 +15,7 @@ does not cover code generated by gccgo).
 
 <p>
 Note that gccgo is not the <code>gc</code> compiler; see
-the <a href="/doc/install.html">Installing Go</a> instructions for that
+the <a href="/doc/install">Installing Go</a> instructions for that
 compiler.
 </p>
 

_content/doc/manage-install.html

@@ -8,8 +8,8 @@ This topic describes how to install multiple versions of Go on the same machine,
 
 <p>For other content on installing, you might be interested in:</p>
 <ul>
-<li><a href="/doc/install.html">Download and install</a> -- The simplest way to get installed and running.</li>
-<li><a href="/doc/install-source.html">Installing Go from source</a> -- How to check out the sources, build them on your own machine, and run them.</li>
+<li><a href="/doc/install">Download and install</a> -- The simplest way to get installed and running.</li>
+<li><a href="/doc/install/source">Installing Go from source</a> -- How to check out the sources, build them on your own machine, and run them.</li>
 </ul>
 
 <h2 id="installing-multiple">Installing multiple Go versions</h2>