docs(storage): add filesystem preparation and preflight guide

[sqlrush]

What this changes

• Add a concrete RHEL 9 x86_64/shared-LVM/GFS2 preparation reference, explicitly not yet PGRAC-qualified across four hosts.
• Document storage prerequisites, configuration results, read-only checks, scratch-only persistent-fd and locking checks, and stop conditions.
• Link from the MVP deployment guide and index; distinguish local four-instance evidence and add corresponding RHEL build dependencies.
• No product code, defaults, workflow, test-harness or immutable tag changes.

How it was tested

• 19 public/private mirrored manuals match byte for byte.
• Relative links, fences, public-boundary checks and 16 shell syntax checks pass.
• Three Python snippets parse; temporary-local-root persistent-fd handoff and exclusive-create/outside-root negative checks pass. Interactive tty was replaced by a pipe for the local check.
• Existing interface coverage: 28 views, 248 columns, 251 registrations, 1038 keys, 33 counters.
• No GFS2 setup, shared-device writes, fencing test, four-host qualification or database workload was run.

Checklist

• Non-cluster behavior unchanged (documentation only; upstream tests not rerun).
• Runtime safety code unchanged.
• Documentation syntax and whitespace checked.
• Documentation updated; no unsupported deployment certification claim.