Docs: 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 documentation.

Config

mkdocs.yml enables the mermaid custom fence under pymdownx.superfences (mirroring the webform/webdav setup already proven on the d.o pages build), 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 to warned to 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.