This replaces and extends the previous commit by using `$var` instead of
`$(var)` when this makes sense and using the ugly workaround for Doxygen
expanding environment variables even inside Markdown backticks elsewhere.
Also use backticks around more strings.
It turns out that Doxygen expands $(VAR) into the actual value of the
VAR environment variable on the machine where it runs, which is
definitely not what we want in the instructions telling people to use
"$wxwin" in their projects, so remove the parentheses to prevent Doxygen
from expanding it.
Another alternative would be to escape "$" with a backslash, but this
would make the source markdown file uglier.
