Improve settings management

[noirbizarre]

Hello 👋🏼

First, thanks for this plugin!

I allowed myself to submit this PR adding multiple improvements to the settings handling:

• remove the unnecessary settings level (in a backward-compatible way)
• add a Neovim health check for it
• simplify the associated documentation (same indent, format... for all example, add syntax highlighting)
• follow lazy.vim recommendations in examples (opts over config)
• add some LuaCATS annotations, so users can benefit from completion and documentation from Lua LSP

Here's the health check for settings:

You can see the rendered README here

[linux-cultist]

Looks like some very good work. :) Will try to find time to go though it this week. Thank you for contributing!