- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

- Sphinx and Doxygen docs now installed by `make install`.

Detect macOS and adapt sed usage in clang-format version check script; also fix small typo in err msg (#169)

Emacs likes to add hidden symlinks with names like '.#myfile.cpp' that don't actually point to real files as a locking mechanism, which cause the format check/apply scripts to fail. This tweaks the format scripts to ignore files that are symlinks (adds a -type f predicate to the find commands), which gets around this issue.