docs: add guide page about lifecycle hooks

[keithamus]

What?

This adds documentation around the lifecycle hooks that custom elements provide.

Why?

This offers some small use to developers, compared to MDN:

• It's easier to find than MDN's pages on this.
• It's more succinct, than an MDN article.
• It is more pragmatic than the MDN article, e.g. saying "you'll almost never need [adoptedCallback]"

Largely though, it gives us a space to document our own developed lifecylce methods. Which we plan to do.

[koddsson]

This looks great 👍🏻