docs(controllers): add DTO folder explanation for full resource sample

[ManavPatel7972]

• 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?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

In the "Full resource sample" section of the Controllers documentation, the example imports the following DTOs:

import { CreateCatDto, UpdateCatDto, ListAllEntities } from './dto';

[kamilmysliwiec]