docs: improve "Intro to Keptn" page

[StackScribe]

Partially satisfies #2005

This PR improves the "Intro to KLT" page:

• Intro page appears before "Getting started" page
• Improve the flow by grouping descriptive information into the three use-cases, which should map to the future modules
• Removed the first sentence that says the section includes tutorials although those old tutorials are still listed here.
• Removed the "GitOps" term, which seemed to be making people who are not doing full GitOps feel that Keptn was not for them
• Add CloudEvents to list of signals Keptn emits
• Remove info about Keptn v1 and the history of Keptn as well as the links to the youtube videos
• Add a sentence that is ready to link to Stacey's blog, which discusses the history of the Keptn project, what Keptn v1 is, and points readers to the youtube videos. A separate PR will insert the link to the blog just before we publish.

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