doc: update security explanation

[elijahgreenstein]

This PR updates the MicroCloud security explanation to align with SSDLC requirements:

• Adds cross references to reference materials on releases and snaps, to direct users to relevant information about supported versions
• Adds links to the MicroCloud security policy and Ubuntu disclosure policy
• Adds a note about logging

Checklist

• I have read the contributing guidelines and attest that all commits in this PR are signed off, cryptographically signed, and follow this project's commit structure.
• I have checked and added or updated relevant documentation.

[roosterfish]

Thanks!