Quickstart bundles manual

This PR does 3 things: 

a) Add `juju quickstart` instructions and video as the preferred method for Getting Started.
b) Add `juju quickstart` instructions to each provider page. Manual configuration is still preserved for each set of instructions.
c) Replace all mentions of "null" provider and replaced it with Manual Provider. 

I was tempted to replace `juju generate-config` with `juju init`, but decided not to. :8ball: