Add link to Crossplane release process from extension release guide

[jbw976]

This PR simply makes the Crossplane release process in https://github.com/crossplane/release more discoverable by adding a hint box to the "Releasing Crossplane Extensions" guide. Folks visiting that guide will now more easily find where to get information about releasing the core Crossplane software, in addition to the extension release information on that page.

This PR is related to graduation due diligence.

[netlify]

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