More docs on WaveDB

[mtanco]

Hello! I used WaveDB for the first time yesterday, and am not a regular SQLite user. I was having really hard time figuring out what the index was for / doing on the connection line in our docs:

from h2o_wave import connect

db = connect()["somerandomtext"]

And just stumbled upon this comment in one of the examples:

wave/py/examples/db.py

Line 25 in 23fb7ae

# A new database is created automatically if it does not exist.

It would be great to have that explained / details on the https://wave.h2o.ai/docs/wavedb/ page!

[mturoci]

Tracked in #1661.