docs(edu): add introduction page as site landing

Adds src/introduction.md explaining the project (LLM-generated learning
resources, Claude Code + Opus 4.6/Sonnet 4.6, CC0 public domain, contact
email) and links it as the first entry in SUMMARY.md.

edu/src/SUMMARY.md

@@ -1,5 +1,7 @@
 # Summary
 
+- [Introduction](introduction.md)
+
 # Markov Chains
 
 - [Markov Chain Course](markov.md)

edu/src/introduction.md

@@ -0,0 +1,26 @@
+# Vibed Learning
+
+Welcome to **Vibed Learning** — a collection of self-guided courses and technical references on programming, computer science, and software engineering.
+
+## What This Is
+
+This site is an experiment in generating custom learning resources using large language models. Every course and reference here was created with [Claude Code](https://claude.ai/claude-code), using Anthropic's **Opus 4.6** and **Sonnet 4.6** models. The goal is to explore how AI-assisted authorship can produce educational content that is clear, accurate, and genuinely useful.
+
+The content is a work in progress. New courses are added as topics are explored, and existing material is revised as needed.
+
+## Content
+
+Browse the table of contents on the left to find courses. Current topics include:
+
+- **Markov Chains** — a self-guided introduction to the math and intuition behind Markov chains
+- **Vector Databases** — how vector search works and when to use it
+- **Git Worktrees** — using multiple working trees with a single Git repository
+- **Writing a Lisp-to-C Compiler in Rust** — building a compiler from scratch
+
+## License
+
+All content on this site is released under [CC0 (Public Domain)](https://creativecommons.org/public-domain/cc0/). You may use, copy, modify, and distribute it freely, without asking for permission and without attribution.
+
+## Contact
+
+Questions, corrections, or suggestions: [vibebooks@elijah.run](mailto:vibebooks@elijah.run)