Consider adding trait-level short_title fallback to title

[caugner]

Context

The PageLike trait's short_title() method returns Option<&str>, requiring every call site to manually fall back to title() via .unwrap_or().

Current behavior

Every call site already applies the same pattern:

• crates/rari-doc/src/helpers/title.rs:68 — root_doc.short_title().unwrap_or(root_doc.title())
• crates/rari-doc/src/helpers/subpages.rs:27-28 — sorting uses .unwrap_or(a.title())
• crates/rari-doc/src/html/links.rs:78 — link rendering uses .unwrap_or(page.title())

Proposal

Add a default method to the PageLike trait (or equivalent) that encapsulates this fallback, reducing repetition and preventing inconsistencies:

fn short_title_or_title(&self) -> &str {
    self.short_title().unwrap_or(self.title())
}

This is a low-priority cleanup item — the current approach works correctly.