[RF-DOCS] Active Record Callbacks

[Ridhwana]

Motivation / Background

This Pull Request has been created to provide more clarity and update the Active Record Callbacks documentation.

Detail

This guide has probably been reviewed multiple times over time, so we can do a quick pass to see where we can make things clearer, simpler, and/or more concise.

Some initial thoughts to get started:

• Samples that use Proc.new {} can likely be simplified to just use proc {}
• Link to the validations guide where we mention it under conditional callbacks
• after_find callbacks are also triggered by a few other methods: take, sole, find_by!
• We could potentially add some more examples on different types of callbacks that aren't there.
• Updated some sections for clarity

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[p8]

I think "Cascading Association Callbacks" is a great title.
These changes are really nice.