Add documentation for Drivers

Warning, you must merge #287 before this pr.
This pr contains the documentation of:
*  `ST_AsKml`
* `ST_AsGeoJson`
* `DBFRead`
* `DBFWrite`
* `GPXRead`
* `GeoJsonRead`
* `GeoJsonWrite`
* `KMLWrite`
* `SHPRead`
* `SHPWrite`
* `file_table`

see #288
You should not be using a specific commit but rather the master branch: https://github.com/irstv/H2GIS/blob/master/h2drivers/src/main/java/org/h2gis/drivers/gpx/GPXRead.java