doc: backport SSDLC updates (v2-edge)

[elijahgreenstein]

Backports PRs #1430, #1449, and #1450 to update the security explanation and add a decommissioning guide to MicroCloud 2.

The security guide was amended as follows:

• Removed references to replicators and cluster links (only valid for LXD 6 and, hence, MicroCloud 3).
• Updated cross references as necessary. For example, the original guide referenced the MicroCeph Best practices for secure deployment guide, but this guide is only in the MicroCloud 3 integrated docs, not MicroCloud 2. In place, a reference to the MicroCeph Security overview was used.

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]

Ty!