This reverts commit 0bb904e10a and
re-introduces the markdown table support extension. The resulting table
by default is too wide in the "responsive" read-the-docs theme, but with
a CSS tweak it becomes readable.
Markdown tables are much easier to deal with and this way we can also
include references within the tables.
Tear it out of the CMakeLists.txt and instead run it via
cargo xtask cppdocs
This allows the build_and_test step in the CI to only run cmake for the
library/header related bits and the docs_and_demos step to only generate
docs and not require a full host build of the library (as cargo xtask
cmake would otherwise do).
