Document how quiet_deps interacts with load paths

[Amoifr]

Closes #62.

@comi91 set quiet_deps: true and still got Bootstrap's deprecation warnings. @stof diagnosed it at the time: Sass only treats a file as belonging to a dependency when it is loaded through a load path — a relative @import into vendor/ makes it your code. The bundle has supported load_path since #37, but the docs still recommend the relative import, so users land straight in the setup where quiet_deps cannot work.

Confirmed against the Sass binary:

Import style	--quiet-deps	Deprecation warnings
relative path	yes	still shown
through a load path	yes	silenced
through a load path	no	shown (control)

So no code change is needed — this documents the interaction: a tip in the Bootstrap section pointing at the load-path approach, and an explanation in Register Additional Load Paths of why it's what makes quiet_deps useful in the first place.

Rendered locally with docs-builder to check the internal link resolves. Touches doc/index.rst like #99, but in different sections — they don't conflict.