Apply the latest Beman Standard

[neatudarius]

Apply the Beman Standard to Beman.Example and more tweaks.

Issues:

• Apply the Beman Standard to beman.exemplar #14
• Add relevant API code example inside Beman.Exemplar #16
• licenses mismatching with Beman Project License #9
• Make build directions work if cut/paste #8

Updates related to the Beman Standard:

• Update licence
  • Apply LICENSE.APACHE_LLVM: Switch from MIT to Apache License v2.0 with LLVM Exceptions.
  • Apply LICENSE.CRITERIA: already applied
  • Apply LICENSE.APPROVED: already applied
• Apply FILE.COPYRIGHT
• Naming
  • Apply LIBRARY_NAMES:
  • Apply REPOSITORY_NAME: renamed to Example (docs + github)
  • Apply TOPLEVEL.CMAKE: already applied (root CMakeLists.txt, using Ctest)
  • Apply TOPLEVEL.LICENSE: renamed file to LICENCE.
  • Apply TOPLEVEL.README
• README
  • Apply README.TITLE
  • Apply README.PURPOSE
  • Apply README.IMPLEMENTS
• CMake
  • Apply CMAKE.DEFAULT
  • Apply CMAKE.SKIP_TESTS
  • Apply CMAKE.AVOID_PASSTHROUGHS
• Tree
  • Apply DIRECTORY.INTERFACE_HEADERS:

    $ tree include 
    include
    └── Beman
        └── Example
            └── example.hpp

  • Apply DIRECTORY.IMPLEMENTATION_HEADERS: currently N/A
  • Apply DIRECTORY.SOURCES:

    $ tree src 
    src
    └── Beman
        └── Example
            ├── CMakeLists.txt
            ├── example.cpp
            └── tests
                ├── CMakeLists.txt
                └── example.t.cpp
    
    4 directories, 4 files

Updates related to docs and usage example:

• Add API example: beman::example::identity equivalent of std::identity
• Add basic unit test and framework integration with Gtest: src/Beman/Example/tests/example.t.cpp.

$ tree src/Beman/Example/tests 
src/Beman/Example/tests
├── CMakeLists.txt
└── example.t.cpp

1 directory, 2 files

• Add usage example: examples/sample_usage.cpp.

$ tree examples 
examples
├── CMakeLists.txt
└── sample_usage.cpp

1 directory, 2 files

• Docs
  • Add build instructions docs.
  • Document minimum compiler requirements
  • Add required tools/packags.
  • Add OS support.
  • Add deps support.
  • Add C++ standard support.

Updates related to CI/infrastructure/other missing bits:

• Extend compiler / standard versions for build and run on CI: {gcc, clang} x {C++98, C++03, C++11, C++14, C++17, C++20, 'C++23, C++26}.
• Add CI Tests badge in root README.md.
• Add .gitignore
• Add .github/CODEOWNERS

[camio]

I wasn't able to add suggestions for the file and directory names, but those should be updated as well.

[camio]

Aside from the TODOs, which can easily be converted into issues against this repository, and a minor issue, this looks good to merge to me.

[camio]

Should the target name be beman.example.tests.identity in case there are multiple test drivers?

[wusatosi]

Extend compiler / standard versions for build and run on CI: {gcc, clang} x {C++98, C++03, C++11, C++14, C++17, C++20, 'C++23, C++26}.

Is removing CI testing for C++98 <-> C++17 here intentional?
There's only C++20 <-> C++26 testing left.

@neatudarius