docs: add warning for webpack

[JustinKeller]

Bundling node_modules with webpack can cause issues.

fix #2806

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• [x] Docs

What is the current behavior?

The serverless FAQ page mentions using webpack to bundle node_modules.

Issue Number: 2806

What is the new behavior?

Warn against bundling node_modules with webpack as it can cause issues.

Does this PR introduce a breaking change?

• [x] No