This is to enable a minimal CI check for the pull requests, to assist
contributors and reviewers, as well as for direct commits and merges.
For each toolset, this runs permutation of
b2 cxxstd=03,11,14,17,2a variant=debug,release address-model=32,64
with cxxstd list tailored according to the actual toolset.
On push, build master and develop.
On pull request, build if any of documentation related files are touched.
Prior to the documentation build, all doc/src/examples are built to
verify valid source code is displayed in the docs.
