SL-xx: Improve docs

[scouillard]

Description

The root README file is getting too large and complex. Add a new docs/ directory and split the documentation into new sections:

• Configuration
• Management - rake tasks
• Management - API

Also in this PR:

• Add Tenants documentation
• Add Servers documentation

Current tree:

- introduction
- before you begin
- architecture
- server requirements
- setup
-   bbb server
-   shared volume for recordings (external)
-   pg database
-   redis   
- deploying scalelite docker containers (external)
- upgrading
- configuration (external)
- management – rake task (external)
- management – API (external)

Testing Steps

Screenshots (if appropriate):