This is more user-friendly than GitHub links with their tons of files.
Also use the recently added #vX.Y.Z_msw anchor to link directly to MSW
binaries downloads from the corresponding file.
Closes https://github.com/wxWidgets/wxWidgets/pull/1820
We don't provide binaries for this compiler any more, just MinGW-w64.
Also mention that SJLJ exceptions propagation mechanism is only used in
32-bit builds.
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.
