Bump the docs version to 8.15.1

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* Rebuild this PR: `run docs-build` * Rebuild this PR and all Elastic docs: `run docs-build rebuild` _{`run docs-build` is much faster than `run docs-build rebuild`. A `rebuild` should only be needed in rare situations.} _{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status [here](https://buildkite.com/elastic/docs-build).}

elastic / docs

Bump the docs version to 8.15.1 #3050