Move demo app to example/ directory

[andreashasse]

The demo handlers (elli_openapi_demo, user_handler) and demo.escript were
mixed into src/ alongside the library code. This separates them into a
standalone example/ app with its own rebar.config (path dep on parent).

Tests still compile elli_openapi_demo via extra_src_dirs in the test profile.
CI now also compiles the example app. README updated to guide users to example/.

https://claude.ai/code/session_018fWKyeGN93ZDdLR3PyW5Yj

[Copilot]

Pull request overview

This PR moves the repository’s runnable demo code out of the main library src/ tree into a standalone example/ application, and updates build/test wiring and docs accordingly.

Changes:

• Remove the root-level demo shell configuration and move demo handlers + script into example/ with its own rebar.config.
• Keep integration tests compiling the demo handler module via extra_src_dirs in the test profile.
• Update CI and README to reflect the new example location and usage.

Reviewed changes

Copilot reviewed 6 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rebar.config	Removes root demo shell config; adds example/src to test profile compilation; updates Hank ignore patterns.
example/src/user_handler.erl	Adds example “user” handler module (demo-only).
example/src/elli_openapi_demo.erl	Adds the primary example OpenAPI handler implementations used by tests and the example app.
example/src/demo.app.src	Defines a minimal OTP application resource for the example app.
example/rebar.config	Adds standalone build config for the example app (path dep on parent).
example/demo.escript	Adds runnable script used by rebar3 shell in the example app.
demo.escript	Removes the old root-level demo script.
README.md	Updates documentation to point users to example/ and adds run instructions + endpoints.
.github/workflows/ci.yml	Adds a CI step intended to compile the example code.

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