Document decoration of schematic routes

It's mentioned in the [schematic FAQ](https://aws-cqc.github.io/DeviceLayout.jl/stable/schematicdriven/faq/) and used inside `add_bridges!` in ExamplePDK, but there's no docstring for `attach!(::RouteComponent, ...)`, and especially no mention of gotchas with respect to route initialization in `plan` and path construction in `build!` (or `SchematicDrivenLayout.path`, which is also undocumented)

Start with the schematic FAQ and the add_bridges! usage in ExamplePDK, then trace attach!(::RouteComponent, ...), route initialization in plan, and path construction in build! and SchematicDrivenLayout.path. Done when the relevant docstrings explain these APIs and their route-initialization and path-construction gotchas.

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