docs: add Russian README translation

[MonteNegroX]

What

Adds a Russian translation of the root README and a language switcher (English · Русский).

Why

Russian-speaking developers are a large audience for this project. Docs-only contribution to improve discoverability.

Scope

• Docs-only (no code/behavior changes)
• Commands, package names, model IDs, and code samples left in English
• Deeper docs links keep English paths where localized docs do not exist yet

How to test

1. Preview the new Russian README on GitHub
2. Click language switcher links
3. Confirm images and install commands match English README

[CLAassistant]

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