Restructure README to clarify what this integration is meant for

[christian-kreuzberger-dtx]

Original Conversation: https://keptn.slack.com/archives/C017T4KUAG3/p1657031728882659

Goal: Job-executor-service needs to better advertise itself as a customization/extension option for Keptn, especially if you're new to Keptn.

Tasks

• Describe on top of README what Job-Executor-Service allows you to do (and not how or why), e.g., subscribe to any Keptn Cloud Event (sh.keptn.event.test.triggered) and run any container/command as a response to this
• Add screenshots, an animated GIF or a YouTube video that quickly show-cases job-executor-service
• This initial pitch should be followed by the quickstart section to get the user started quickly, with a link to detailed installation instructions in docs/INSTALL.md
• Refactor Motivations table into prose and describe some of the capabilities of Job-Executor