Add additional Spark usage guides to documentation

## Context

Two additional usage guides for Apache Spark with CrateDB should be added to the Spark integration documentation page.

## Usage Guides to Add

1. Machine Learning on the Edge with Apache Spark: https://github.com/crate/crate-ml/blob/master/docs/ml_on_the_edge/apache_spark/Readme.md (by @patrickFuerst)
2. PySpark Write DataFrame to CrateDB via JDBC: https://github.com/crate/pre-sales/blob/master/Demos/PySpark/pyspark_write_data_frame_to_CrateDB_jdbc.py (by @ckurze)

## Target File

`docs/integrate/spark/index.md`

## References

- Pull Request: https://github.com/crate/cratedb-guide/pull/281
- Comment: https://github.com/crate/cratedb-guide/pull/281#discussion_r2388862038
- Requested by: @amotl