Bump ecctl version tp 1.15.0

[nick-benoit]

Updates docs to reference latest ecctl version

Rel: elastic/ecctl#727

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[nick-benoit]

QQ for the docs folks. The release guide in the ecctl repository says the following:

Once the PR for the above changes has been merged, you'll need to run a full doc rebuild to make the new release branch the default docs.

The link to the full doc rebuild seems to be broken now though. Is there a new process for this now?