Issue #3596698: add Mermaid diagrams for the key model and main flows

Make the key model and the main flows easier to follow by adding Mermaid diagrams to the docs.

Config: mkdocs.yml enables the mermaid custom fence under pymdownx.superfences (mirroring the setup already proven on sibling modules' d.o pages builds), so fenced mermaid blocks render as diagrams.

Diagrams added:

• concepts.md: the three-tier key hierarchy and the passphrase two-layer wrap (converted from the existing ASCII art), plus a new unlock-flow sequence (derive with Argon2id, unwrap, hold the Subject KEK in a sliding-TTL session).
• cross-site-api.md: the consumer/vault architecture (converted from ASCII) and a cross-site read sequence (OAuth token plus opaque handle, the ConsumerApi grant/trust gate, authorized vs uniform 403).
• dormant-vaults.md: the active -> warned -> erased lifecycle as a state diagram, including the reset-on-activity edge.
• multi-tenancy.md: per-tenant isolation (one account, two independently-encrypted vaults under separate Master KEKs).

Command and config snippets stay as code; only genuine diagrams were added. Every block was validated against the mermaid engine, and mkdocs build --strict passes.