Improve server-side rendering documentation (#2178)

* Improve server-side rendering documentation

* Fix typo

* Document Node + mhchem

* Re-order require statements

Co-authored-by: ylemkimon <mail@ylem.kim>

docs/node.md

@@ -75,4 +75,25 @@ import katex from 'katex/dist/katex.mjs'
 ```
 
 > The ES module contains ES6 syntaxes and features, and may need transpiling to
-use in old environments:
+use in old environments.
+
+See the [API](api.html) for available functions in the `katex` module.
+
+## Including in webpages
+
+To render any HTML generated by e.g. `katex.renderToString`, you will
+still need to link the CSS file, make the KaTeX font files available to the
+client, and use the HTML5 doctype.  See [browser usage](browser.html).
+Note, however, that you do not need to include `katex.js` on the client.
+
+## Using mhchem extension
+
+The [mhchem extension](https://github.com/Khan/KaTeX/tree/master/contrib/mhchem)
+adds its features by modifying the `katex` module.  So you can use KaTeX with
+mhchem in Node as follows:
+
+```js
+const katex = require('katex');
+require('katex/dist/contrib/mhchem.js'); // modify katex module
+const html = katex.renderToString('\\ce{CO2 + C -> 2 C0}");
+```