docs: restructure dataset schema page with introduction and guidance (#2554)

## Summary

Restructures the dataset schema documentation to provide proper context
before diving into details:

- Adds introduction explaining what dataset schema is and its two
components (`fields` and `views`)
- Moves file structure section before examples so readers know where to
put code
- Reorganizes content into clear Fields and Views sections as parallel
concepts
- Adds guidance on why/when to use views (addressing user feedback)
- Documents what views are NOT for (anti-patterns, export format
misconceptions)
- Adds multi-view example for different use cases (Marketing vs Pricing)
- Consolidates reference tables at the end
- Links to Google Maps Scraper as real-world example

The page now follows the same pattern as other actor definition pages
(input_schema, output_schema).

## Context

Based on feedback from Martin Sabo and Jaroslav Hejlek in #dev-docs -
the documentation explained HOW to configure views but not WHY or WHEN
to use them.

Slack thread:
https://apify.slack.com/archives/C010Q0FBYG3/p1779357816377359?thread_ts=1779115904.940779&cid=C010Q0FBYG3

## Test plan

- [ ] Verify page renders correctly in preview
- [ ] Check all code examples are valid JSON
- [ ] Verify Google Maps Scraper link works
- [ ] Confirm new headings appear in table of contents

https://claude.ai/code/session_01JyTmwWUsZaN7436BBgxwvG

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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Michał Olender <92638966+TC-MO@users.noreply.github.com>

Author: 3 people