bref: idiomatic layer layout + Lambda deploy example & docs

Align the Lambda artifacts with Bref's own convention (as used by
brefphp/extra-php-extensions) so a release drops in like any other Bref
extension and the same binary works for both layer and Docker deploys:

- Layer zip now ships bref/extensions/quickjs.so (Bref's extension_dir) and
  bref/etc/php/conf.d/ext-quickjs.ini with a bare `extension=quickjs.so`,
  instead of an /opt/php-quickjs/*.so with an absolute path.
- Add examples/lambda/ — a runnable skeleton: a Bref function handler that
  runs a TS guest, a Dockerfile (FROM bref/php-84:3, copies the .so into
  /opt/bref/extensions), and a serverless.yml covering both the container
  image and the layer route.
- Expand docs/install.md's AWS Lambda section to document all three paths
  (Docker image, Lambda layer, vendored .so) and how Bref loads extensions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R3vgA3Q6PR9VQn8X5pLcMR

docs: slim the top-level README to a high-level overview

Make the README read like a normal repo landing page: short intro, a
features list, one quick example, an installation pointer, a brief
"how it works", and links down into the docs. Move the full API reference
out to docs/api.md and drop the project-layout block (the docs source map
already covers it).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R3vgA3Q6PR9VQn8X5pLcMR