Improve AWS integration docs

[rishabhpoddar]

• [ ] Add a note saying that the apiDomain value should be your aws lambda url in “backend config” section (at the end)
• [ ] Add note about testing in the docs that we don’t support it.
• [ ] In Setting up API gateway, first point about creating base path /auth, we should mention that we need to create a resource /auth
• [ ] When adding the middleware, add it to the index.js and not auth.js file.
• [x] We should change the snippets to work with .mjs files
• [ ] Mention that API domain should be changed to the URL on which you reach the lambda function
• [ ] Mention that if getting 502, see logs and also check for timeout setting in aws lambda
• [ ] In “Using session verification” section, change auth.ts to some other file name.
• [ ] In your lambda functions, make sure to set statusCode explicitly to 200 cause middy sends 500 by default.