Keptn 0.11.x Tutorials Update + Quickstart guide + New Learning path

[oleg-nenashev]

This pull request is a bulk update that produces a new version with the following goals

• Update the Keptn Tutorials for the 0.11.x release which was missing before
• Make 0.11.x the tutorial displayed by default. Was 0.9.x before
• Link the Tutorials site to the public quickstart Guide on the Keptn website
• Introduce a new learning path when tutorials start from quickstarts: Local or a Public Demo. Only after that a full tour is offered

This is a temporary implementation for the Quickstart tutorial, later a new one will be introduced along with Antora

Screenshots

Default landing:

Quickstart landing:

Quickstart tutorial:

Depends on

• Minor updates for the Keptn tutorial keptn.github.io#983
• fix: Update Quickstart SLI config to produce proper Configuration YAML in the Prometheus Service examples#215
• Quickstart: Add sections and hyperlink anchors keptn.github.io#988

[johannes-b]

LGTM

[oleg-nenashev]

I will merge it and address issues incrementally. Thanks @johannes-b !