Update documentation

[pk910]

Reminder to keep the documentation up to date!

Missing stuff:

• schedule section in test configuration & test scheduler description
• coordinator section in test configuration with general settings (test retention time & execution concurrency)
• existence of API, api.enabled setting & reference to swagger api documentation within the assertoor UI
• enhance description of variable system (global / test config is not passed down to all task configs)
• describe configVars and the ability to use jq syntax for more complex expressions

Nice to have:

• Description of assertoor code structure
• How to use assertoor as go library (call tasks, call tests)