[documentation] write docs

[mgrzybek]

We should find:

• internals and technical architecture ;
• how to install the appliances into an existing project ;
• how to send data èto the tools ;
• how to use the tools.

[fredmj]

Maybe also a plan for the doc. A plan splitted into chapters and with an order of reading :
The main entry point of the doc could be the Readme.md. With a link to description.md and so on ...

0. Index
1. Chapter 1
2. Chapter 2
   2-a. parahraph a
   ... etc

[mgrzybek]

I've just added a file : https://github.com/139bercy/cloud-appliance-observability/blob/master/docs/README.md

Pandoc can create a table of content but it's quite difficult to deal with splitted markdown files.