feat: Don't bundle api-examples in wheel distribution

[cpsievert]

Partly addresses #2125

This PR removes the shiny/api-examples/ directory from the distributed package (both wheels and source distributions) to reduce package size and file count. The examples are only needed during documentation builds, which always run from the source repository.

Motivation

The api-examples/ directory contains ~310 example files used by the @add_example() decorator to inject code examples into docstrings during documentation generation. These files:

• Are only accessed when SHINY_ADD_EXAMPLES=true (set during make docs-quartodoc)
• Are never needed by end users installing the package
• Added ~400KB to the package size
• Contributed ~36% of the total file count in the wheel

Similar to development dependencies, these are build-time artifacts that don't belong in the runtime package.

Changes

1. MANIFEST.in

- recursive-include shiny/api-examples *
+ prune shiny/api-examples
+ prune shiny/experimental/api-examples

Why: Controls what goes into source distributions (.tar.gz). The prune directive explicitly excludes the directories.

2. pyproject.toml

 [tool.setuptools]
-packages = { find = { include = ["shiny", "shiny.*"] } }
+[tool.setuptools.packages.find]
+where = ["."]
+include = ["shiny*"]
+namespaces = false
+
+[tool.setuptools.exclude-package-data]
+"*" = ["api-examples/**"]

Why: Controls what goes into binary wheels (.whl):

1. packages.find: Expanded from inline syntax to dedicated section for better control
2. exclude-package-data: Explicit blacklist for api-examples as additional safeguard

Why both MANIFEST.in and pyproject.toml?

These control different build artifacts:

• MANIFEST.in → Source distributions (.tar.gz) - used by some users, required by PyPI
• pyproject.toml → Binary wheels (.whl) - used by most users via pip install

The wheel build process doesn't read MANIFEST.in; it uses setuptools configuration from pyproject.toml. We need to exclude api-examples from both to ensure they don't appear in either distribution format.

3. shiny/_docstring.py

def find_api_examples_dir(start_dir: str) -> Optional[str]:
    # ... search for api-examples ...

    # Not found - provide helpful error for documentation builders
    if os.getenv("SHINY_ADD_EXAMPLES") == "true":
        raise FileNotFoundError(
            "Could not find 'api-examples/' directory. "
            "Documentation must be built from the source repository, "
            "not from an installed package. "
            f"Searched from: {search_start}"
        )
    return None

Why: Provides a clear error message if someone attempts to build docs from an installed package instead of from source.