Document the Scrapy component API

[Gallaecio]

There are certain classes in Scrapy that may optionally define the from_crawler and from_settings class methods.

At the moment, we document this fact in every single one of them. We repeat ourselves a lot. But we also often fail to cover how to properly subclass a class that defines one of these methods.

I think we need to extend the documentation with a section or a page about this type of classes, and have all such classes reference this central documentation instead of providing their own documentation about this aspect of such classes.

[OrestisKan]

Hi, @Gallaecio seems like this issue is still relevant, right? If so I would like to work on it.

[Gallaecio]

Thanks!