Document the new parameter file support for components and controllers

[SprGrf]

From the discussion under this PR, @eeberhard's orginal comment:

I think somewhere we will need to define 1: the schema of the parameters file, 2: the priority order of parameter resolution, and 3: the meanings of the various path specifiers.

For 1, is there a URL to a ROS standard (or at least an example) we can link to?

For 2, the order is: component default parameters from implementation - overridden by -> parameters file - overridden by -> individual parameters from the parameters property

For 3, file:// is an absolute path in the container, data:// is a path relative to the default mounted data folder in the container, and package:// is a path relative to a ROS 2 package installed in the container.

But mostly, can we include all of this information in the description of the property in a concise way? It seems too much. So I think it would be good to clearly define these points in a public issue or docs page and link to that instead.

So I guess we need to decide where this information could be included (under core controllers/controllers in documentation maybe?).

fyi: @domire8 @eeberhard