docs: add onboarding guide to gh-pages

[kamir]

Summary

• Add _docs/onboarding-guide.md — single-page guide covering the E10–E50 progressive tutorial path and 101 developer guide
• Update _docs/index.md with one new bullet in the "Get started" section

Companion to PR #130 (code examples targeting main).

Content sections

• Overview, prerequisites, and learning path table
• E10: Java Kafka Client — produce/consume basics, KafScale-specific config
• E20: Spring Boot — REST API, profiles, Docker deployment
• E30: Apache Flink — stateful streaming, 3 deployment modes
• E40: Apache Spark — structured streaming, DataFrame API, checkpoints
• E50: JavaScript — Node.js client, web UI, agent integration
• Common configuration — deployment profiles table, env vars
• Developer guide overview (101 chapters)
• Next steps — links to quickstart, user-guide, architecture

Verification

• Front matter matches conventions (layout: doc, nav_order: 4, permalink: /onboarding-guide/)
• Apache 2.0 HTML comment license header present
• index.md has exactly one new bullet added
• Internal links resolve (/quickstart/, /user-guide/, /architecture/)
• GitHub source links point to correct main branch paths

Test plan

• Verify page renders correctly on gh-pages (Jekyll build)
• Confirm navigation order is correct (after User Guide, before Installation)
• Check all internal and external links resolve

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