Add additional Spark usage guides to documentation

[coderabbitai]

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: Databricks and Spark: Basic usage guides #281
• Comment: Databricks and Spark: Basic usage guides #281 (comment)
• Requested by: @amotl