docs(contributing): add instructions on building @zod/docs (#5114)

This adds instructions on how to build `@zod/docs` and start a dev server.

The necessary token was not obvious, so I thought I might save someone else the trouble.

CONTRIBUTING.md

@@ -31,6 +31,26 @@ The following steps will get you setup to contribute changes to this repo:
 
 4. Start playing with the code! You can do some simple experimentation in [`play.ts`](play.ts) (see `pnpm play` below) or start implementing a feature right away.
 
+### Building Docs Locally
+
+#### Dev Server
+
+To start a dev server, run:
+
+```sh
+pnpm run --filter=@zod/docs dev
+```
+
+#### Production Build
+
+To build `@zod/docs` for production, you will need to set the `GITHUB_TOKEN` environment variable to a personal access token. [Create a granular personal access token](https://github.com/settings/personal-access-tokens/new) and accept the defaults (no extra permissions are necessary). Then:
+
+```sh
+export GITHUB_TOKEN=your_token_here # persists in shell session
+pnpm run --filter=@zod/docs build
+```
+
+> The `GITHUB_TOKEN` environment variable is used to fetch stargazer counts of projects in Zod's ecosystem.
 
 ## Alternative: VSCode Dev Container setup