enh(stream-connector): attempt to improve stream connector onboarding

[omercier]

Description

Changes were made on two aspects:

In Broker BBDO documentation

• some tables were in HTML, others in improper markdown format, it was painful to edit them, so I tidied it up a little
• host and service status events are the most used in LUA stream connector develeopment but their fields lacked a description, so I completed it as much as I could.

In the stream connector specific doc & tutorial

• changed the plan to something more didactic
• tried to propose more relevant scripts for each step in the progression
• use broker cache API v2
• removed some lua language related hints that I found were too numerous

I'm not fully happy with this but I hope it actually improves the documentation.

Target serie

• [x] 20.04.x
• [x] 20.10.x (master)