Documentation: Populate fuseml welcome documentation page

[stefannica]

The introductory part of the fuseml documentation (https://fuseml.github.io/docs/) needs to cover at a minimum:

• fuseml project overview (what it is, what it does, what problems is it trying to solve)
• main fuseml concepts (artifacts, runnables, codesets, worflows and workflow templates)
• high-level modus operandi (how to use it)

[stefannica]

Covered by fuseml/docs#2