Allow exclusions when extracting example code

[mingxwa]

Allow passing more arguments to extract_example_code_from_docs.py to exclude specific files in case some features, like modules, are only available in the most recent compiler versions. This change also supports multiple code blocks in one .md file. No functional changes. Tested manually.

[SidneyCogdill]

I'm working on a much more advanced solution:

• Allow custom CMake template for each example targets (per doc target). The usecase can be easily addressed using a template like this:

if(PROXY_BUILDING_WITH_MODULE)
  add_executable($NAME$ code_2.cpp)
  
  target_link_libraries($NAME$ msft_proxy::module)
  target_compile_features($NAME$ PRIVATE cxx_std_20)

  target_sources($NAME$ PRIVATE
    FILE_SET CXX_MODULES
    FILES code_1.cpp
    )
endif()

• Multiple codeblocks are supported and added to per doc target, unlike current script which rejects more than 1. This is useful to showcase the module support (importing and exporting)

you can close this and wait for my implementation (I'm still testing it).