Create new guidance from SF.12 to prefer "" where possible

[apenn-msft]

That makes sense, split it into separate guidance:
1: if you use #include "alpha.h", alpha.h must exist at a relative path
2: if you can include alpha.h from a relative path, prefer to; this will increase the likelihood the correct file is located

Originally posted by @apenn-msft in #1664

[apenn-msft]

This would require changing the current SF.12 which was just reviewed and merged. I'm partial to just leaving it as-is since the points are highly related and easier to comprehend if treated in the same rule.