UART. Dossmatik's UART and unisim guides. We have *.doc sources to be converted to Sphinx.
Coverage. Code coverage in GHDL is a side effect of using GCC as a backend. In the future, GCC backend support might be dropped in favour of mcode and LLVM. To do so, code coverage with LLVM should be supported first. Anyway, comments/bits of info should be gathered somewhere in the docs, along with references to gcov, lcov, etc.
It is possible to add waveforms in the docs with wavedrom, since there is a sphinx extension available.