Add translator attribution alongside the existing authors option

[mmcky]

Part of QuantEcon/meta#360 (author and translator attribution across all themes).

What

The theme already supports an authors option in html_theme_options — a list of {name, url} pairs rendered per lecture page. Add a parallel translators option with the same shape, rendered alongside the authors, so translated editions (zh-cn, fa, fr) can credit the people who produced them.

Scope for this repo

• Accept translators in html_theme_options with the same {name, url} list shape as authors.
• Render translators next to the existing authors block under the page title, visually distinguished from authors (distinct class, rel="author" only on authors).
• Allow per-page front matter to override both authors and translators for that page.
• Make the "Authors" / "Translators" labels overridable so each edition can render them in its own language.
• Omit the translators block entirely when the list is empty, so original-language editions are unaffected.

Design decisions

The data model, placement, and label-localisation decisions are being settled in the tracking issue QuantEcon/meta#360 so all three themes stay consistent — implement against whatever is agreed there rather than deciding locally.

Acceptance

• A lecture repo with translators configured shows the translator credits on every page; one without it renders exactly as today.
• A page-level override replaces the project-level value for that page only.