Add some (but not all) docs for the Rust crate #1669

Job	Run time
build-benchmarks	3m 8s
Lint and Test	13m 55s
build-with-external-dependencies (-F gdal -F gdal/bindgen)	1m 5s
build	29s
build-with-external-dependencies (-F geos)	44s
build (-F csv)	24s
build-with-external-dependencies (-F proj)	1m 10s
build (-F flatgeobuf)	24s
build (-F flatgeobuf_async)	37s
build (-F geozero)	20s
build (-F ipc_compression)	26s
build (-F parquet)	26s
build (-F parquet_async)	32s
build (-F parquet_compression)	34s
build (-F polylabel)	24s
build (-F postgis)	42s
build (-F rayon)	30s
	25m 50s

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