docs: discourage usage of weak functions

As a coding guideline, we now discourage introducing new weak
functions in platform-agnostic code going forward and provide the
rationale for this.

This was already enforced most of the time in code reviews but this
patch makes it explicit in the project's documentation.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I88f4a55788899fb4146c4d26afb3a7418376b30c