docs: translate the site nav to Japanese and unify the footer

[Naruto]

What

The Japanese docs rendered the English nav: labels verbatim. The i18n plugin
translates navigation only when the ja locale carries a nav_translations map,
and this repo had none — page bodies were already fully translated, so the menu
was the last English surface on the ja site.

• Japanese menu — nav_translations covering every nav entry. Labels come
  from each page's own H1, and match the wording SSPlayerForUnity already uses
  wherever the same English title appears in both repos.
• Emoji — on every top-level nav entry and on each page's H1, following the
  SpriteStudio-Docs portal. Child entries stay emoji-free, as they do there.
• Footer — the portal's footer: year-less copyright plus the GitHub and
  OPTPiX SpriteStudio icons, with Japanese icon labels on the ja build.

Sibling PRs apply the same change to the other four docs sites, so all six
(including the portal) now share one nav and footer convention.

Notes

nav_translations keys are matched literally, so they carry the emoji too. A key
that stops matching its nav: title falls back to the English label silently
— it does not fail the build — so the two lists have to move together.

Adding an emoji to an H1 prepends a hyphen to that heading's anchor id
(#-license). The portal already generates ids that way, and nothing in this
repo links to an H1 anchor, so no existing link changes.

Test plan

• mkdocs build --strict — no warnings
• Translated N navigation elements to 'ja' equals the total nav entry count
• Rendered ja nav has no English labels left, proper nouns aside
• Footer shows the copyright and both icons in each locale
• Language switcher still resolves per-page after the ja extra override