docs: add site-messages README

[brian-smith-tcril]

What

Adds src/i18n/site-messages/README.md documenting the operator-managed local translations directory.

Why

The frontend-base i18n pipeline added in openedx/frontend-base#205 (part of openedx/frontend-base#176) introduced src/i18n/site-messages/ as the checked-in, operator-managed location for site-local translation JSON files. The layout in this repo is:

src/i18n/
├── index.js              ← checked in, re-exports from messages.ts
├── messages.d.ts         ← checked in, TypeScript type stub
├── messages.ts           ← gitignored, generated by translations:prepare
├── messages/             ← gitignored, populated by atlas pull
└── site-messages/        ← checked in, operator-managed local translations
    ├── README.md
    └── *.json            ← operator's own translations; merged in last (override behavior)

(The frontend-base migration guide documents this re-export module as index.ts; this repo landed it as index.js in #11 — that discrepancy is out of scope here.)

translations:prepare generates a gitignored site-messages/index.ts from the JSON files an operator drops in this directory, and those strings are merged in last so they can override strings pulled from packages. The repo's .gitignore already references src/i18n/site-messages/index.ts, but the directory itself and its explanatory README were never committed — the README only ever existed in the throwaway testing PR #10, which was closed without merging.

This adds the README so the directory exists in a fresh clone and operators understand what to put there. The content matches what was in PR #10.

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