Fix rustdoc link errors on rustc 1.99 and nightly

[SomeB1oody]

Problem

The doc workflow fails on master. The workflow runs cargo doc with RUSTDOCFLAGS="-D warnings" on the latest stable toolchain. Stable moved to rustc 1.99.0 on 2026-09-28.

Rustdoc 1.99 resolves a link in a module doc (//!) in the scope of that module. An inline link such as [`Optimizer`](crate::neural_network::traits::Optimizer) then fails rustdoc::redundant_explicit_links, because the label alone resolves to the same item. Nightly (1.101) flags more of these links, so stable 1.101 will break the workflow again.

The build docs of GroupNormalization and InstanceNormalization (from #5) also link the private MIN_RANK constant. This fails rustdoc::private_intra_doc_links.

Reproduction

Run on master (403042f):

RUSTDOCFLAGS="-D warnings" cargo +1.99.0 doc --all-features --no-deps

error: public documentation for `build` links to private item `MIN_RANK`
   --> src/neural_network/layers/regularization/normalization/group_normalization.rs:255:11
    |
255 |     /// [`MIN_RANK`]
    |           ^^^^^^^^ this item is private
    |
    = note: this link will resolve properly if you pass `--document-private-items`
    = note: `-D rustdoc::private-intra-doc-links` implied by `-D warnings`

error: public documentation for `build` links to private item `MIN_RANK`
   --> src/neural_network/layers/regularization/normalization/instance_normalization.rs:254:11

error: redundant explicit link target
  |
  = note: when a link's destination is not specified,
          the label is used to resolve intra-doc links
  = note: `-D rustdoc::redundant-explicit-links` implied by `-D warnings`

error: could not document `rustyml`

Fix

• Module docs use reference-style link definitions: [`Optimizer`] in the text and //! [`Optimizer`]: crate::neural_network::traits::Optimizer at the end of the block. All 128 inline crate-path links in 41 module docs use this form now, not only the links that 1.99 flags.
• Removing the target alone is not correct. Rustdoc 1.89 (the MSRV in Cargo.toml) cannot resolve these labels in a module doc. The reference-style form resolves on all 3 toolchains.
• The build docs of GroupNormalization and InstanceNormalization give the rank value (2 and 3) in place of the private link.
• The outer doc of neural_network::error writes Error as code. The module doc of that module already links the type.

No code changes.