docs: add DSL Language Guide + wire the three-doc DSL learning path

[rdevaul]

The yapCAD DSL had a reference (function/type catalog) and a tutorial (one worked example), but no document teaching the language AS a language — its mental model, execution semantics, and idioms. Newcomers had no on-ramp and no 'how to think in this DSL' guide; the why/how lived scattered across index.rst prose and historical specs.

Adds docs/dsl_guide.md — a DSL Language Guide covering:

• what the DSL is and why it exists (type safety, guaranteed termination, provenance, automation/LLM-friendliness)
• the execution model (module -> command -> emit -> CLI)
• UPPERCASE-export vs lowercase-helper command convention
• thinking in static types; the angle-units convention (degrees for geometry/transforms, radians for trig); control flow and no-while design
• the build/transform/boolean composition workflow
• idioms & gotchas (over-cut subtractions, 0.0 - x negation, degrees/radians mismatch, require contracts, aggregation over chained unions, radial patterns)
• @meta/@ui annotations and the CLI lifecycle

Documents the degrees-vs-radians convention that was previously only implicit in per-function comments: transform/geometry functions (rotate, rotate_2d, arc, ellipse) take DEGREES; trig (sin/cos/tan) takes RADIANS. Added prominent callouts to the Guide (§4 + gotchas), the Reference (Transformation + Math section headers), and the Tutorial (Tips).

Also wires the three docs into a clear learning path:

• index.rst toctree reordered to Guide -> Tutorial -> Reference
• reciprocal cross-links added to the Tutorial and Reference headers/footers

Depends on the myst_parser fix (PR #40, merged) for the Markdown to render. Docs build verified clean (no dsl_guide warnings) against main.

Co-authored-by: jarvis-rich