Documenting satisfying trait bounds and candidate preference

My goal is to document the changes from #120752.

This requires a lot of interdependent concepts, so it's currently a fairly big change. How do you want me to proceed here? Concretely, I would like to get input on the general structure here before discussing the specifics of this doc.

The things in bold are what I am am adding/have to add to document things. What does this rely on:

• items
  • instantiating generic parameters of an item
• types
  • alias types (associated types + opaque types)
    • normalizing associated types
    • the concept of a rigid alias
  • inference variables
  • placeholders
• type equality
  • structural except for alias types, what does structural mean, concept of a rigid type
  • relating aliases
  • higher ranked
• trait bounds
  • satisfaction: how to prove/satisfy trait bounds
    • via trait implementations
    • via in-scope where-bound
    • via item bounds of rigid aliases
    • candidate preference (what I originally set out to document)

Ways in which these things are interdependent:

• equality of alias types needs to know about normalization
• normalization needs to know about "satisfying trait bounds" as its behavior only makes sense in reference to that
• satisfying trait bounds needs to talk about equality
• using item bounds relies on the concept of rigid aliases

Additional dependencies and annoyances:

• to explain how we use impls to satisfy a trait bound, I currently rely on inference variables and the concept of "instantiating generic parameters when using an item"
• to explain equality and subtyping of higher ranked types, I am talking about inference variables and placeholders

I don't mind splitting this up into smaller changes and keeping things either as TODO or just not documenting things, e.g. we could just never explain what it means to "equate a type" until that gets merged separately or ignore satisfying trait bounds via item bounds of rigid aliases.

What I would like to know is:

• does this structure seem good? Working on the concrete docs feels unpleasant while that's still up in the air
• how do I take this structure and actually get it into the reference?

A WIP branch is in https://github.com/rust-lang/project-goal-reference-expansion/pull/7/files. Started a zulip convo for this.

Start by reviewing the WIP branch in project-goal-reference-expansion PR #7 and the linked Zulip discussion about the trait solver and trait bounds. Agree on the documentation structure and whether to split the concepts into smaller changes; done means the structure is settled and the relevant material is added to the Rust Reference.