Documentation for DevWorkpace Operator specific features

[sleshchenko]

DevWorkspace provides a few features on the top of devfile:

• mounting secrets/config maps into devworkspace;
• configuring image pull secrets;
• configuring git https tokens;
• configuring devworkspace on namespace level;
• annotate the devworkspace to debug startup;

Features list is growing on the way we're moving forward integration with Che, and the only place where these features are described - go code.
It's needed to start DevWorkspace Operator Documentation to describe such features, which will be the source of truth for Che and other dependent projects.

From docs team:

a documentation built using Antora and following modular docs guidelines, like what we have for devfile or che-docs would be perfect and much appreciated in the eventuality of a future downstream reuse.
that a new docs project would be best to start using the toolchain we already have in place for Che and devfile docs

[JPinkney]

Initial devworkspace configuration documentation branch is: https://github.com/devfile/devworkspace-operator-docs/tree/configuration-documentation