feature, add standalone examples directory

[steven-varga]

Add a dedicated examples/ directory with small, standalone, copy-pasteable C++ programs demonstrating each libdecimal representation and common use cases.

Motivation:
The documentation already contains embedded snippets, but there is no examples/ folder with standalone programs that a user can build and run immediately. This is a gap for onboarding and for verifying that find_package(libdecimal) works end-to-end.

Proposed scope:
Create examples/ with one program per representation plus a CMake integration example:

• examples/bid_example.cpp — IEEE 754-2008 BID: construction, UDLs, transcendentals, formatting
• examples/fixed_example.cpp — compile-time fixed-point: currency-style arithmetic, constexpr, decompose
• examples/bcd_example.cpp — BCD: string construction, digit-level access, str()
• examples/scaled_example.cpp — scaled integer pair: wire-format style, as_pair(), checksum
• examples/CMakeLists.txt — optional target libdecimal_examples built when libdecimal_BUILD_EXAMPLES=ON

Build integration:
Add a top-level option:

option(libdecimal_BUILD_EXAMPLES "Build example programs" OFF)

When ON, add the examples/ subdirectory and build each example linked against libdecimal::libdecimal.

Acceptance criteria:

• examples/ directory exists with the four example programs listed above.
• cmake -Dlibdecimal_BUILD_EXAMPLES=ON ... && cmake --build ... compiles all examples.
• Each example runs successfully and prints clear output.
• Default build (libdecimal_BUILD_EXAMPLES=OFF) does not build examples.
• Examples use the same coding style as the rest of the project (C++23, project naming conventions).