Reorganize and bundle dataframe resources for pandas, Dask, and Polars with SQLAlchemy

## Description

Reorganize and bundle documentation resources that share the same ingredients between pandas, Dask, and Polars, specifically when using SQLAlchemy for data imports.

## Motivation

Currently, there is redundancy in the documentation and examples across different dataframe libraries (pandas, Dask, Polars) when they use the same underlying mechanisms (SQLAlchemy, `insert_bulk` utility). This creates maintenance overhead and can lead to inconsistencies.

## Goals

- Decrease redundancy by consolidating common patterns and documentation
- Generalize across all items of the same category
- Make it easier to maintain and update documentation for all three dataframe libraries
- Improve user experience by providing a unified approach to bulk data imports

## Context

This issue was identified during review of dataframe examples in cratedb-examples repository.

**Related PR**: https://github.com/crate/cratedb-examples/pull/1202
**Discussion**: https://github.com/crate/cratedb-examples/pull/1202#discussion_r2488246527
**Requested by**: @amotl