feat: add comprehensive documentation for Mastra project setup and migration

- Created `create-mastra.md` to provide a complete guide for creating new Mastra projects, including quickstart CLI methods and detailed manual installation steps.
- Introduced `embedded-docs.md` to explain how to look up API signatures from embedded documentation in `node_modules/@mastra/*/dist/docs/`, emphasizing version accuracy and local availability.
- Added `migration-guide.md` to outline the process for upgrading Mastra versions, including checking official migration docs, using embedded docs for current APIs, and common migration patterns.
- Developed `remote-docs.md` to guide users on accessing current documentation from the Mastra website when local packages are unavailable, detailing the documentation structure and lookup workflows.