docs: add GitHub buttons and improve option docs

docs/themes/navy/layout/index.swig

@@ -3,6 +3,7 @@
     <h2 id="banner-title">{{ page.subtitle }}</h2>
     <div id="banner-start">
       <code id="banner-start-command">npm install liquidjs</code><a id="banner-start-link" href="./tutorials/setup.html"><i class="icon-arrow-right"></i></a>
+      <a id="banner-github-link" href="https://github.com/{{ config.github }}" target="_blank" rel="noreferrer">GitHub</a>
     </div>
     <div id="banner-share">{{ partial('partial/share') }}</div>
   </div>

docs/themes/navy/layout/partial/after_footer.swig

@@ -5,6 +5,7 @@
 
 {{ js('js/main') }}
 
+<script async defer src="https://buttons.github.io/buttons.js"></script>
 <script src="https://cdn.jsdelivr.net/npm/docsearch.js@2/dist/cdn/docsearch.min.js"></script>
 <script>
 document.getElementById('search-input-wrap').classList.add('on');

docs/themes/navy/layout/partial/header.swig

@@ -18,7 +18,10 @@
         {% endfor %}
       </select>
     </div>
-    <a href="https://github.com/{{ config.github }}" class="main-nav-link icon-nav-link"><i class="icon-github"></i> <span class="icon-nav-title">GitHub</span></a>
+    <div class="main-nav-link icon-nav-link github-buttons-wrap">
+      <a class="github-button" href="https://github.com/harttle/liquidjs" data-color-scheme="no-preference: light; light: light; dark: dark;" data-size="large" data-show-count="true" aria-label="Star harttle/liquidjs on GitHub">Star</a>
+      <a class="github-button" href="https://github.com/sponsors/harttle" data-color-scheme="no-preference: light; light: light; dark: dark;" data-size="large" aria-label="Sponsor @harttle on GitHub">Sponsor</a>
+    </div>
     <a id="mobile-nav-toggle">
       <span class="mobile-nav-toggle-bar"></span>
       <span class="mobile-nav-toggle-bar"></span>