ci: extend the rustdoc gate to the whole workspace

[justin13888]

What is wrong

#399 added mise run doc-check-rust — RUSTDOCFLAGS="-D warnings" cargo doc --no-deps — and
wired it into check-rust, but scoped it to the four crates #399 declared frozen:
capsule-core, capsule-core-ffi, capsule-wasm, capsule-i18n.

The rest of the workspace is still ungated, and it is not clean. Measured at f433d918,
before #399's fixes:

crate	error spans under -D warnings
capsule-server	12
capsule-sdk	8
capsule-cli	1
capsule-wire	1
xtask	1

They are the usual classes: unresolved intra-doc links, redundant explicit link targets,
public documentation linking to a private item, and ambiguous links where a name is both a
function and a module.

Until they are fixed the gate cannot be widened, and a broken doc link in those crates is
merged without anything noticing.

What should land

• Fix the 23 spans.
• Widen [tasks.doc-check-rust] in mise.toml from the explicit -p list to
  cargo doc --workspace --no-deps (dropping the per-crate flags entirely, so a new crate
  is gated the day it is added).

Acceptance

RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps exits 0, and mise.toml's
doc-check-rust names no crate.

Note capsule-wire is scheduled for retirement in #400; if that lands first its span
disappears with it.

Refs #399.