docs: add new API route guides + fixes and improvements to existing ones (#8987)

- Add new documentation pages for API route intro, responses, errors, and validation
- General fixes and improvements across existing API route guides

> Note: I didn't work on the guide for additional data validation. Will work on this as part of my next focus on workflows.

www/apps/book/app/advanced-development/api-routes/http-methods/page.mdx

@@ -20,7 +20,7 @@ import type {
   MedusaResponse,
 } from "@medusajs/medusa"
 
-export const GET = (
+export const GET = async (
   req: MedusaRequest,
   res: MedusaResponse
 ) => {
@@ -29,7 +29,7 @@ export const GET = (
   })
 }
 
-export const POST = (
+export const POST = async (
   req: MedusaRequest,
   res: MedusaResponse
 ) => {