feat(outline): make resolve_named_destination public

[ajbufort]

What

Make PdfDocument::resolve_named_destination public (it was pub(crate)), with a
doctest.

Why

Named destinations (ISO 32000-1 §12.3.2.3 / §7.9.6) are referenced by string from
Link/GoTo actions and outline items. The crate already resolves a name to a 0-based
page index - get_outline() uses it internally for named outline dests - but the
resolver is not reachable, so a downstream consumer that follows link targets (or
wants to resolve a name it obtained some other way) has to re-walk the catalog
/Dests dict + /Names → /Dests name tree itself, duplicating logic that already
lives here and is tested.

Exposing it lets a consumer turn any named destination into the page it targets with
one call, so e.g. a /Link annotation whose action is GoTo a named dest can be
rendered as an internal page jump.

Change

• resolve_named_destination(&self, name: &str) -> Result<Option<usize>>:
  pub(crate) → pub.
• Adds a # Example doctest (this repo requires doctests on public items).

No behavior change; purely a visibility + documentation change.

Checklist

• cargo build
• cargo clippy -- -D warnings
• cargo test --doc resolve_named_destination (the new doctest)
• cargo fmt
• DCO sign-off