Improve supported functions documentation (#1511)

* Remove empty lower right cells

* Escape backticks(`) using <code></code>

* Escape vertical bars(|) using <code> and HTML entity code

* Normalize documentation links

* Remove empty thead

website/empty_thead.js

@@ -0,0 +1,30 @@
+module.exports = function(md, options) {
+    function removeEmptyThead(state) {
+        const tokens = state.tokens;
+
+        let thead = -1;
+        let empty = true;
+        for (let i = tokens.length - 1; i >= 0; i--) {
+            const tok = tokens[i];
+            switch (tok.type) {
+                case 'thead_close':
+                    thead = i;
+                    break;
+                case 'thead_open':
+                    if (empty) {
+                        tokens.splice(i, thead - i + 1);
+                    }
+                    thead = -1;
+                    empty = true;
+                    break;
+                case 'inline':
+                    if (thead !== -1 && tok.content.length > 0) {
+                        empty = false;
+                    }
+                    break;
+            }
+        }
+    }
+
+    md.core.ruler.after('block', 'empty_thead', removeEmptyThead);
+};