Improve dim-si README for new users

[dyreby]

The dim-si README has a few gaps that new users hit early.

The Quick Start should be the primary onboarding path. Rather than adding separate sections for each concept, expand the Quick Start to demonstrate more operations (multiplication, division, scaling) using the fluent q() API — that's the style we want users to reach for first. Free functions get a brief mention, not a full code block.

Quick Start structure

Walk through a small coherent scenario rather than isolated snippets:

1. Create quantities from values — show kilometer(100), hour(2), etc. and call out that this is how you wrap raw numbers (e.g., from JSON or external APIs)
2. Compute — use q() fluent API to combine them: division for speed, addition with mixed units
3. Extract values — show .in(unit) to get plain numbers back out, explain valueIn as the non-fluent alternative
4. Compile-time safety — show a dimension mismatch error
5. Affine units — brief temperature example since it has different semantics

Free functions get a one-liner mention at the end.

README section order

1. Installation
2. Quick Start (above)
3. Type Annotations — how to write functions with Linear/Affine (see Make dim-si types easy to use for end users #7)
4. Units (reference table)
5. Custom Scaled Units
6. Adding a New Unit (contributor-facing)

Other improvements

• Ensure import consistency (everything comes from @isentropic/dim-si subpaths, not lower-level packages)

[dyreby]

PRs ready for review:

• Add type annotation and compound unit docs to dim-si README #5 (type annotations + compound units) — can merge independently
• Add worked example to dim-si README #6 (worked example) — stacked on Add type annotation and compound unit docs to dim-si README #5, merge that first

[dyreby]

@gdtroszak Ready for review. Start with #7 and #8 for context, then #5 is ready for review. #6 is stacked on #5.

[gdtroszak]

Proposed plan for the README update:

Structure

1. Intro blurb (keep as-is)
2. Installation (keep as-is)
3. Quick Start — 4 subsections with brief connective prose between them:
   • Create and compute — unit functions, fluent q() API arithmetic, dimensions carrying through
   • Extract values — .in(unit) as primary, one-liner mention of valueIn
   • Type safety — dimension mismatch compile errors, typed function signatures with Linear/Affine, affine unit semantics
   • Custom units — prefixes, scale composition
4. Units (reference tables, keep as-is)
5. Adding a New Unit (contributor-facing, keep as-is)
6. License

What changes from the current README

• Custom Scaled Units standalone section folded into Quick Start
• Free functions standalone code block removed; brief mention only
• .in(unit) replaces valueIn as the primary extraction API in examples
• Type annotations added under type safety (depends on Make dim-si types easy to use for end users #7)