fix cargo doc E0080 on MSRV 1.85 (p256/p384/p521/sm2/k256)

[Warfront1]

Fixes #1909.

cargo doc fails on the published p256/p384/p521/sm2/k256 0.14.x crates when building with Rust 1.85-1.89 (E0080 at src/arithmetic/tables.rs).

This adds #[doc(hidden)] to the offending associated const in all five affected crates (p256, p384, p521, sm2, k256).
Hiding the const from rustdoc prevents the const evaluator from running on it, resolving E0080 while keeping MSRV 1.85.
The const remains usable at runtime; only its doc rendering is suppressed.

docker run --rm rust:1.85.0 bash -c '
  apt-get update -qq && apt-get install -y -qq git >/dev/null
  git clone --depth 1 --branch feat/fix-doc-e0080-rust140653 https://github.com/Warfront1/elliptic-curves.git /repo
  cd /repo
  cargo doc --workspace --all-features
'

The underlying rustc issue (rust-lang/rust#140653) is fixed in Rust 1.90 (rust-lang/rust#140942).
When MSRV is bumped to 1.90, this #[doc(hidden)] workaround can be removed.