[boost] Code coverage reports

Still in development, but actually the first iteration is functional. Run coverage reports using gcovr + github pages. No Codecov. To explain more: imagine Codecov-like coverage reports, except they are built with an open source software tool "gcovr". Hosted on Github Pages, and you have potentially more control over the output, the steps, the appearance, etc. since the script is customizable. Instructions: https://github.com/boostorg/boost-ci/blob/master/docs/code-coverage.md