Added section on the declaration order of subject, let!/let and before/after hooks

[schinery]

Added section on the declaration order of subject, let!/let and before/after hooks to the Example Group Structure section.

Also, moved [Leading Subject] to the Example Group Structure section, as mentioned below.

Resolves https://github.com/rubocop/rspec-style-guide/issues/134

[schinery]

Thanks for handling this!

Suggestions:

1. Keep the existing leading-subject guideline as is. There are external resources linking to guidelines, and we don’t want to break them. There’s a workaround to specify a legance anchor name.
2. I’d move both guidelines to the example group structure section.
3. Let’s call them “hooks”

@pirj hoping I've done these suggestions correctly. Please let me know if this needs any more amends.

[pirj]

Thank you, @schinery !