Add 'pg_autoctl inspect fsm mermaid' phase diagrams for the keeper FSM

[dimitri]

What

Replaces the hand-maintained Graphviz FSM diagram (docs/fsm.png, regenerated by hand via pg_autoctl inspect fsm gv | dot, last touched in 2021) with five smaller diagrams generated directly from KeeperFSM[] (src/bin/pg_autoctl/fsm.c), so they can never drift out of sync with the actual state machine.

The full FSM is 20 states and 77 transitions — legible as a reference table, but too dense to read at a glance as a single diagram. New commands split it by phase of a node's life:

pg_autoctl inspect fsm mermaid init
pg_autoctl inspect fsm mermaid steady-state
pg_autoctl inspect fsm mermaid failover
pg_autoctl inspect fsm mermaid maintenance
pg_autoctl inspect fsm mermaid removal

Each prints a Mermaid stateDiagram-v2 program, embedded directly in docs/failover-state-machine.rst via sphinxcontrib.mermaid (already a docs dependency).

Design notes

• Phase classification is a small, hand-curated (current, assigned) -> phase table in fsm_mermaid.c, verified at runtime: every transition lands in exactly one diagram (a log_warn fires if a future KeeperFSM[] change ever adds an edge the table doesn't cover — nothing silently disappears).
• Citus doesn't get a 6th diagram. Verified all 15 Citus-specific (current, assigned) pairs already exist under NODE_KIND_ANY — a separate Citus diagram would be graph-isomorphic to "failover," so it's called out as a note there instead.
• join_primary is excluded. It's long deprecated and no longer assigned to any node (see the existing "Join_primary" doc section); KeeperFSM[] still carries its 9 transitions for backward compatibility with on-disk state from old versions, but showing them just adds noise. 77 edges -> 68 shown.
• States shared across diagrams are explicit, not hidden. Every diagram annotates each of its states with note right of X naming every other phase that state also appears in.
• Colour scheme via Mermaid classDef/class, consistent across all five: grey (lifecycle/admin), blue (primary-like), green (secondary-like), red (primary going away), purple (maintenance), amber (multi-standby election machinery).
• pg_autoctl inspect fsm gv (Graphviz) is left in place for anyone who wants the single full graph (join_primary included) for their own tooling — it's just no longer the documented way to visualize the FSM.

Verification

• Native build (make -C src/bin/pg_autoctl) clean, no warnings.
• All 5 inspect fsm mermaid <phase> commands run and produce valid Mermaid syntax — caught and fixed one real bug along the way: a colon inside note text collides with Mermaid's note right of X : text delimiter grammar (verified against mermaid@11.12.1 directly).
• Docs built with Sphinx and the rendered HTML checked in a browser: all 5 diagrams render without errors, colours and cross-diagram notes display correctly, join_primary confirmed absent everywhere.
• Edge-count cross-check: 68 edges total across the 5 diagrams, matching 77 real KeeperFSM[] transitions minus the 9 involving join_primary, with zero dropped and zero duplicated.