Split Validations and Callbacks into separate guides.

[steveklabnik]

Two big features that are only barely related in the same guide. Seems
bad.

I did not check references to these guides yet, so some links may need
to be updated.

[steveklabnik]

/cc @fxn @vijaydev

[carlosantoniodasilva]

Seems a good change to me.

[richo]

Seems reasonable, as a user the SEO impact longer term will be noticable.

When I first starting trying to learn rails on the job with an existing codebase, there were a lot of "wat" moments when some info was buried in a seemingly unrelated page. Seems like a good step in the right direction.

[fxn]

👍

[rafaelfranca]

👍, needs to update the guides list. https://github.com/rails/rails/blob/master/guides/source/documents.yaml

And add a Changelog entry.

[steveklabnik]

🤘