Deprecate and retire PSModule/docs after consolidation cutover

[MariusStorhaug]

Parent: #423
Blocked by: #426

After documentation and issue migration, PSModule/docs should be deprecated and retired.

Request

Execute repository retirement with a controlled cutover and explicit communication.

Acceptance criteria

• PSModule/docs clearly marked deprecated before retirement.
• Readme and key entry points point to Process-PSModule as the active hub.
• Archive or delete action is performed after migration validation.
• Parent issue Consolidate PSModule documentation into Process-PSModule and retire PSModule/docs #423 contains final retirement checklist and completion note.

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Technical decisions

Safety-first retirement: Deprecation notice period precedes archival/removal.

User guidance: Landing page continues to direct users to current docs location.

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Implementation plan

• Add deprecation notice and migration pointer
• Validate all critical docs links resolve to Process-PSModule
• Archive or remove PSModule/docs per org decision
• Record completion and close parent Consolidate PSModule documentation into Process-PSModule and retire PSModule/docs #423

[MariusStorhaug]

The source cleanup PR PSModule/docs#112 is merged, but https://psmodule.io/docs/ still serves the legacy Pages artifact. Do not close this task or archive the source repository until an edge/reverse-proxy mapping preserves /docs/* paths to the Process-PSModule site and live canonical links are validated.