Add embed tool for embedding files in markdown

[lentzi90]

This adds a plugin to mdbook for embedding files from any repo on github.
My plan is to use this when refreshing the quick-start guide. To see how it works, check this WIP PR: #585

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[lentzi90]

I removed the phony for releasetags since it seemed unnecessary to me. We were not using it anywhere so the only thing it could be used for is to manually build the binary releasetags.

[lentzi90]

/cc @peppi-lotta
This should be useful when creating tests for the quick-start. We can embed whatever manifests we use for the tests directly in the guide. That way we always keep them in sync 🙂