New C++ Api: Initial setup of Api documentation. (#6295)

New C++ Api: Initial setup of Api documentation. (#6295)

This configures the initial setup for generating Api documentation with
Sphinx via Breathe and Doxygen. All fixes in the documentation of the
cvc5.h header are for the purpose of eliminating warnings. This PR does
not check for completeness of the documentation, and does not yet tweak
the documentation to be nice, beautiful and consistent, which is
postponed to future PRs.

Configure with `--docs`, and then make. This will generate a `docs`
directory in the build directory. The Sphinx documentation can be found
at `build/docs/sphinx/index.html`. Doxygen documentation is only
generated as xml under `build/docs/doxygen`.

This PR further proposes a new style for copyright headers. If this
style is approved, I will submit a PR to update the update_copyright.pl
script.