Simplified and updated embeddings doc

[jmatthiesen]

Made some changes to simplify the document and focus on the core details of what embeddings are, leaving other documents to describe the details of using them.

• Re-ordered the content so it's:
  • Description of concept
  • Example use cases
  • Summary of how you'd use embeddings with links off to appropriate docs

Removed some links to Semantic Kernel content that was a bit confusing or not necessarily relevant (e.g. Prompt Template Syntax).

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