docs: make README the Kazakh-NLP stack hub (badges, index)

[stukenov]

What this does

Repositions the README so kazakh-nlp-toolkit reads as the umbrella index / entry point for the whole open Kazakh-language AI portfolio, while preserving all existing content and the original voice.

Changes

• Badge row — License (MIT), Python 3.8+, and a Kazakh-language badge.
• One-line value prop positioning this repo as the entry point to a full Kazakh-language AI stack.
• New ## The Kazakh-language AI stack section — a markdown table indexing the sibling projects with one-line descriptions and links:
  • slm, turkic-tts, kazakh-speech-pipeline, qazlang, kazakh-video-dubbing, video-lingo-kazakh, kazakh-micro-lm, nanochat-kazakh, data-translate

Notes

• Marketing / OSS-discoverability pass only — no code changes, no LICENSE changes.
• This repo's own tool descriptions (translation, training, datasets, utils) were left intact and verified against the actual scripts; nothing about this repo's contents was misrepresented.

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