Misc cleanup to representing types and generic parameter defs section

[BoxyUwU]

Some small stuff: flattens some very nested chapters into only one layer of nesting, replace some old terminology with new terminolgoy, add a note to the early/late bound chapters warning ppl that they talk about some advanced stuff that was not yet covered. also removes the section on representing constants since it really doesn't go into much detail at all, is somewhat out of date, and talks a lot about generic_const_exprs stuff which isn't super important to talk about

[jieyouxu]

Thanks!

[marxin]

@BoxyUwU

Can you please fix the following 2 orphan links:

error: File not found: ../early-late-bound-params/early-late-bound-summary.md
   ┌─ ty_module/param_ty_const_regions.md:13:177
   │
13 │ - [`TyKind::Param`]/[`ConstKind::Param`]/[`RegionKind::EarlyParam`] for early bound generic parameters (note: all type and const parameters are considered early bound, see the [chapter on early vs late bound parameters][ch_early_late_bound] for more information)
   │                                                                                                                                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../early-late-bound-params/early-late-bound-summary.md

error: File not found: ../early-late-bound-params/early-late-bound-summary.md
   ┌─ ty_module/param_ty_const_regions.md:68:251
   │
68 │ In contrast to `Ty`/`Const`'s `Param` singular `Param` variant, lifetimes have two variants for representing region parameters: [`RegionKind::EarlyParam`] and [`RegionKind::LateParam`]. The reason for this is due to function's distinguishing between [early and late bound parameters](../early-late-bound-params/early-late-bound-summary.md) which is discussed in an earlier chapter (see link).
   │                                                                                                                                                                                                                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../early-late-bound-params/early-late-bound-summary.md