docs: record system.importLegacy removal (re-apply #4035)

[panghy]

Re-applies the protocol-docs removal of system.importLegacy by reverting commit 787c1ed (#4078). The content mirrors the original docs removal #4035, adjusted to current main.

HELD DRAFT — do not mark ready, do not merge. Parked until the feature is confirmed removable; tracking issue: #4082.

Docs refresh needed before merge

This is a pure revert-of-the-revert: it also removes the 9.1 restoration records from docs/protocol/versioning.md, leaving the docs describing 9.0 as latest. Since 9.1 has been released, the eventual removal is a new breaking change: before merge, refresh these docs to record the removal as protocol 10.0 while keeping the 9.0 removal and 9.1 restoration history entries.

History

• Removal originally shipped in intentd 0.9.0 / cloudlands-fe alpha v2.123.0 (protocol 9.0; docs landed as docs: record system.importLegacy removal (protocol 9.0) #4035).
• Restored as protocol 9.1 (docs recorded in docs: record system.importLegacy restoration (protocol 9.1) #4078).

Merge order for the removal: cloudlands-fe → intentd → this docs PR (last).