Add info on how to run the docs locally

Closes GH-2297.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: slorber

docs/community/contribute.server.mdx

@@ -34,6 +34,14 @@ As a user you’re perfect for helping us improve our docs.
 Typo corrections, error fixes, better explanations, new examples, etcetera.
 All MDX docs live in `docs/`.
 
+You can run the docs locally by following those steps:
+
+*   Use Node.js 14
+*   Install latest npm (not the one coming with Node.js 14): `npm install -g npm`
+*   Install repository packages: `npm install`
+*   Build the docs site: `npm run docs-prod`
+*   Test the static docs site: `npx serve public`
+
 ### Improve issues
 
 Some issues lack information, aren’t reproducible, or are just incorrect.