Add a prominent link to the standard library docs (#16)

Motivation: The standard library docs are some of the most important
docs but currently very hard to find. I've noticed that even fairly
advanced users have never heard of them and complain about "zero docs".

Author: odenix

src/supplemental-ui/partials/header-content.hbs

@@ -51,6 +51,7 @@
           <a href="{{siteRootPath}}/main/current/resources.html" class="navbar-link">Resources</a>
           <div class="navbar-dropdown">
             <a class="navbar-item" href="https://github.com/apple/pkl">GitHub</a>
+            <a class="navbar-item" href="https://pkl-lang.org/package-docs/pkl/current/index.html">Standard Library</a>
             <a class="navbar-item" href="https://pkl-lang.org/package-docs/">Package Docs</a>
             <a class="navbar-item" href="{{siteRootPath}}/main/current/style-guide/index.html">Style Guide</a>
             <a class="navbar-item" href="{{siteRootPath}}/security.html">Security</a>