docs: add layer styleguide
docs: add layer styleguide

Add a first draft of a meta-nilrt layer styleguide, to coordinate syntax
and styling.

Signed-off-by: Alex Stewart <04cdb2793e4fb0120cc30d1c823099f3862701f4@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt