Adds a dedicated concept page documenting how composition variables work end-to-end, from declaration to runtime resolution.
## What's covered
- Declaring variables via `data-composition-variables` on the `<html>` root — full schema with all 5 types (`string`, `number`, `color`, `boolean`, `enum`) and their type-specific options
- Reading resolved values in composition scripts with `__hyperframes.getVariables()`
- Per-instance overrides via `data-variable-values` on host elements (sub-composition embeds)
- CLI overrides via `--variables` / `--variables-file` and `--strict-variables` for strict validation
- Layering/precedence table showing how the three sources merge
- Lint and runtime validation (what undeclared/type-mismatch/enum-out-of-range mean)
- Programmatic access via `extractCompositionMetadata()` for tooling authors
Also adds the page to the Concepts nav group in `docs.json`.
ukimsanov