Check for unused .inc files

[OskarStark]

Follows symfony/symfony-docs#15336

[iamvar]

I am not sure that this check should be part of doctor-rst as rst files itself are valid.
Maybe this should be separate github action in symfony-docs that checks such cases.
I used simple bash script to find them.

[OskarStark]

Sure but DOCtor ist not about valid or not but about linting.

Let's say we want to ensure only one blank line max, 2 blank lines are valid too.

So why shouldn't this fit here?

[iamvar]

It can, but from my perspective, it is a separate thing.
You can try to fit it there.
Moreover rst does not specify which extension should included file be, it is .inc in Symfony case, but theoretically, it can be .rts

[OskarStark]

We can make the extension it configurable.

We have nearly the same rule where we want to report LinkDefinitions which are not used anymore. It's some kind of dead code and should not be maintained anymore/in the future.