docs: add jwt schema docs (#4867)

Author: colinhacks

packages/docs/content/api.mdx

@@ -232,6 +232,7 @@ z.hostname();
 z.emoji();         // validates a single emoji character
 z.base64();
 z.base64url();
+z.jwt();
 z.nanoid();
 z.cuid();
 z.cuid2();
@@ -493,6 +494,15 @@ const cidrv6 = z.string().cidrv6();
 cidrv6.parse("2001:db8::/32"); // ✅
 ```
 
+### JWTs
+
+Validate [JSON Web Tokens](https://jwt.io/).
+
+```ts
+z.jwt();
+z.jwt({ alg: "HS256" });
+```
+
 ## Template literals
 
 > **New in Zod 4**