Add tips to troubleshooting page

[karenfeng]

Users often accidentally make the following mistakes, which we should address in the troubleshooting page:

• Installing glow with Pypi, rather than glow.py
• In v1.0.0: failing to set the SparkSession during registration (i.e. spark = glow.register(spark) rather than glow.register(spark)). This may also require detaching and reattaching to the cluster.

[bboutkov]

Thanks @karenfeng!

For the record, the second bullet caused the following error during block_variants phase of WGR

Undefined function: 'assert_true_or_error'. This function is neither a registered temporary function nor a permanent function registered in the database 'default'.; line 1 pos 7