docs: expand README with usage, status, and org wordmark

[OmarAlJarrah]

Summary

The README had fallen behind the code. It was a bare design-doc index written before the pipeline existed: it listed only four of the six design docs, described nothing runnable, and predated the compilers → passes → emitters vocabulary now used throughout the repo.

This rewrite brings it up to date and aligns it with the README conventions used across the org's SDK repos.

• Reflect the current state: the ir package and OpenAPI 3.x compiler are implemented and driven end-to-end by morphic compile; emitters are not built yet. An honest status callout says so, and a milestone table tracks what's shipped versus planned.
• Add Install and Usage — the CLI (flags, exit codes, IR-JSON-to-stdout behaviour) and a library snippet through the engine package.
• Add the enforced package-layout table, the full design-doc set (including emitter-design.md and reference-learnings.md, both previously missing), and the Building commands that mirror the CI gate.
• Add the theme-aware dexpace wordmark header, badge row (gate CI, license, Go version, coverage), and a contents nav, matching the layout used in the other dexpace SDK repos. The wordmark SVGs are byte-identical to the shared org assets.

Docs only — no code changes.

Test plan

• Verified the library snippet matches the real engine API (New() (*Engine, error), Run(ctx, path, RunOptions{}) (*Result, error), Result.Document / .Diagnostics).
• Verified the CLI flags, usage string, and exit codes against cmd/morphic/compile.go.
• Confirmed every linked doc path exists under docs/.
• Wordmark renders light/dark via <picture> + prefers-color-scheme.

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