Add link to new documentation page

Author: ychin

index.html

@@ -33,6 +33,8 @@ <h2 class="project-tagline">Vim - the text editor - for macOS</h2>
 <li>
 <a href="https://github.com/macvim-dev/macvim/releases">Releases</a>: Binary releases and release notes</li>
 <li>
+<a href="/docs/gui_mac.txt.html">Documentation</a>: MacVim documentation</li>
+<li>
 <a href="https://github.com/macvim-dev/macvim/wiki/FAQ">FAQ</a>: Answers to some frequently asked questions</li>
 <li>
 <a href="https://github.com/macvim-dev/macvim/wiki/Troubleshooting">Troubleshooting</a>: How to track down the source of common problems</li>