CMS-024L — Update PGN editorial workflow documentation

[levallem]

Summary

• update the PGN feature description to include reopening saved project positions
• document the complete current PGN editorial round trip
• clarify that reopened saved positions can continue normal PGN ply navigation on the read-only central board

Motivation

The previous wording described saved positions only as a read-only list and did not explain that a saved position can be reopened for continued PGN review.

Changes

File	Change
README.md	Document saved-position recovery, reopening, saved-ply navigation, read-only central board, and independence from the original PGN file.

PR type

• Documentation only (type:docs)

Validation

• git diff --check ✅
• documentation reviewed against the current behavior on main
• No tests/build run; documentation-only change

Screenshots

Not applicable; no visual changes.

Database / migration impact

None.

Scope

Documentation-only change; no source code, tests, schema, migrations, translations, or Cargo changes. No production, Docker, MongoDB, VPS, .env, or deployment changes.

Closes #30