docs: unify app wiring documentation

[julienrbrt]

Description

Generalize the documentation of app wiring between modules.

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[tac0turtle]

will this be expanded? not sure it needs its own file, not entirely sure why it was added to the docs as well?

[julienrbrt]

I actually added it in every PR, I thought it might help if you want to have an app with only specific modules. You tell me if not needed, we can use this PR to remove from everywhere.

[tac0turtle]

I could be wrong, but this is related to how testing will be done which will have its own documentation. Right now the spec describes module specific designs.

Would a user need to look at this file? trying to better understand when someone would look at it

[julienrbrt]

What will be the content of these files, I guess, still can be defined. Maybe it should be under Building Modules instead, I have not put much thought into it. I do think we should mention it in the documentation, as it shows what are the dependencies of a module (that's why I thought specs was a good place).