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.
• 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.