Improve documentation

[jeddy3]

(If time is short, please review #4592 over over this pull request so that we can address #3514.)

Follow up from #4561

I went through the overhauled docs again now that they are live on the website.

I want to make sure the:

• sidebars worked well
• content was a clear as it could be when view on the website

In this pull request I:

• updated some code examples to be more complete
• removed duplication in the steps we link to from the saved replies
• updated the saved replies to use stylelint.io
• added missing info to Node.js API returned promise
• used lists to more cleanly show the different effects of some options on the CLI vs Node.js API
• standardised list punctuation
• fixed links to awesome stylelint
• adding a couple of extra examples here and there for better context
• finalised the division between what rule conventions exist in the user guide and which go in the developer guide
• fixed some heading hierarchies and their content to better support the website sidebar

[ybiquitous]

LGTM! 👍

(I left some nitpicks, but you don't always need to address them.)

[jeddy3]

@ybiquitous Thanks for suggestions. I accepted them both.