docs(windows): add documentation for 'treat hardware deadkeys as plain keys'

A 'hardware deadkey' is a deadkey that's implemented on the base Windows keyboard, e.g. on Windows' German keyboard. mcompile can treat those as either a deadkey, so you press e.g. ^, a --> â and then the Keyman keyboard can have a rule + 'â' > 'something', or as a plain key, so that '^' deadkey just generates '^'. Generally, that's a better behaviour for mnemonic layouts, as punctuation keys are often overloaded, as in sil_ipa.

It's all very complicated!

See also the example at:
https://community.software.sil.org/t/building-a-keyboard-to-work-with-spanish-deadkeys/4639/3

I see that the documentation is missing an entry for this! https://help.keyman.com/products/windows/18.0/basic/config/options

So ... we should fix that.