docs(rig-lancedb): Improve instructions and example

[cvauclair]

The rig-lancedb documentation could be improved with the following:

• Document that lancedb requires the user to have installed protoc on their machine (the lancedb library will not compile otherwise) and add instructions on how to install it for a couple of popular systems.
• In the examples, when the table is created, add a check to see if the table already exists. The current examples will fail if run twice since the table creation step will error with a "table already exists" error.