Document and lock down types in bevy_ecs::archetype (#6742)

# Objective
Document `bevy_ecs::archetype` and and declutter the public documentation for the module by making types non-`pub`.

Addresses #3362 for `bevy_ecs::archetype`.

## Solution
 - Add module level documentation.
 - Add type and API level documentation for all public facing types.
 - Make `ArchetypeId`, `ArchetypeGeneration`, and `ArchetypeComponentId` truly opaque IDs that are not publicly constructable. 
 - Make `AddBundle` non-pub, make `Edges::get_add_bundle` return a `Option<ArchetypeId>` and fork the existing function into `Edges::get_add_bundle_internal`.
 - Remove `pub(crate)` on fields that have a corresponding pub accessor function.
 - Removed the `Archetypes: Default` impl, opting for a `pub(crate) fn new` alternative instead.

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## Changelog
Added: `ArchetypeGeneration` now implements `Ord` and `PartialOrd`.
Removed: `Archetypes`'s `Default` implementation.
Removed: `Archetype::new` and `Archetype::is_empty`.
Removed: `ArchetypeId::new` and `ArchetypeId::value`.
Removed: `ArchetypeGeneration::value`
Removed: `ArchetypeIdentity`.
Removed: `ArchetypeComponentId::new` and `ArchetypeComponentId::value`.
Removed: `AddBundle`. `Edges::get_add_bundle` now returns `Option<ArchetypeId>`