Initial docs for const generics

[BoxyUwU]

This is a fairly minimal set of const generics docs. It doesn't talk much about min_generic_const_args but definitely should. It also doesn't talk much about adt_const_params but definitely should :3 though we mostly already have docs for that living in hackmd for one of the lang meetings. Just need to upstream those which I intend to do as part of drafting an RFC for adt_const_params

cc @lcnr added a new type system invariant for the whole const eval determinism thing, in some sense this is a more specific version of other stuff like "normalizing assoc types should yield a single type" but it feels interesting enough to list it independently.

cc #2663

[rustbot]

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