Add concrete documentation guidelines for contributors (operations, guides, SampleHelper)

Write how to write different topics (operation docs, guides) and use SampleHelper (https://github.com/Kotlin/dataframe/blob/master/samples/README.md#samplehelper)

### Problem

There are no clear rules for writing documentation (KDocs, guides, examples), so contributions are inconsistent and hard to review.

### Expected

Define a single guideline for writing documentation.

### Acceptance criteria

- A guideline document is added to the repo
- It covers:
- KDocs for operations
- guides / topics structure
- usage of SampleHelper
- Includes at least 2 concrete examples (good/bad or before/after)
- All new doc tasks can reference this guideline

### Motivation

- Multiple contributors are writing docs
- Lack of standards slows reviews and leads to inconsistent quality
- Clear rules are required before 1.0