feat(site): onboard docs-engine, publishing doc/ as the guide

[burinc]

ebb had no docs/ directory at all — unlike raylib-android (which had full
scaffolding, just disabled while the repo was private), this is a
from-scratch onboarding.

doc/evaluation.md, doc/conformance.md and doc/adr/001-fiber-affinity.md
are real, reader-oriented documentation — but they're also load-bearing:
test/ebb/conformance_test.clj hardcodes "doc/divergences.edn" and
"doc/conformance.md" as its gate, and nine files under src/ebb/impl/
carry See doc/conformance.md comments. Moving them would mean rewriting
that gate and a dozen-plus source comments, so instead
docs/guide/{evaluation,conformance}.md and
docs/guide/adr/001-fiber-affinity.md are relative symlinks into doc/
— same pattern as awesome-jolt's guide/index.md → README.md. doc/
stays the one canonical path every test and source comment already points
at; the site gets a window into it, not a copy.

docs/guide/index.md is a new page (not a symlink) — ebb's README is
already a GitHub-facing pitch, so the guide gets its own overview that
indexes the three real pages, matching glitter's README-vs-guide split.

Known gap, not fixed here

conformance.md's own prose links to divergences.edn (the machine-readable
half of the same registry). That's an .edn file, not a doc page the engine
renders, and publishing it as a site asset would mean rewriting that link in
the canonical file the test suite reads verbatim — not worth doing just to
suit the site's layout. That one link 404s on the published site
specifically; it's fine on GitHub, and the guide page's own prose says so
and points there instead.

Verified locally

Against docs-engine v0.2.0:

• bb build succeeds, docs/check-site.sh passes (index, guide overview,
  evaluation, conformance and the ADR all present).
• Served the build under /ebb/ (not root) — all 10 emitted URLs return
  200, nothing escapes the base path, no unrendered {{site-base}}.
• Confirmed the cross-links inherited from evaluation.md/conformance.md/
  the ADR (written assuming doc/'s own flat-plus-adr/ layout) correctly
  rewrite to .html at the same nesting depth under docs/guide/.
• Confirmed the divergences.edn gap is exactly that gap and nothing else
  (a clean 404, not a broader breakage).

Not done here

GitHub Pages has no site at all for this repo yet. Enabling it is its own
explicit step, done right before merge per the org's own migration runbook,
not bundled into this PR.