Improve docs for output schema

[patrikbraborec]

Problem

It is hard to grasp and understand the output schema, especially for inexperienced developers with Apify.

Solution

Improve the docs and add more hands-on examples on how to write correct input & output schemas.

• https://docs.apify.com/platform/actors/development/actor-definition/input-schema
• https://docs.apify.com/platform/actors/development/actor-definition/dataset-schema
• https://docs.apify.com/platform/actors/development/actor-definition/key-value-store-schema

Also, check the consistency of naming (output schema, dataset schema, etc.) across the docs.

[jancurn]

We can reuse some text from https://github.com/apify/actor-whitepaper?tab=readme-ov-file#basic-concepts

[TC-MO]

Multiple improvements landed: 1976, 2197, 2215, 2317, 2254