Mintlify migration: sync with master, full Japanese translation, bilingual navigation

[trev-org]

Updates the mintlify-docs/ folder with the completed Mintlify migration. All changes are contained inside mintlify-docs/ — mintlify-docs/LICENSE, mintlify-docs/scripts/, and everything outside the folder are untouched.

What's included

Synced with master

All content changes on master since the migration snapshot (55ff35c1) are ported into the MDX pages:

• 36 pages updated (news tutorials rewritten for Vespa Cloud, autoscaling examples rework, custom-overrides/pod-template rework, mTLS certificate rotation guide, batch update examples, new zones, new rank features, fast forest evaluation for LightGBM/XGBoost, and more)
• Added: en/security/host-images (new upstream page) and en/operations/az (availability zones page that was missed in the original migration snapshot), including their images
• Removed: the four deleted Kubernetes pages (local-deployment, ecr-pull-through-cache, dev-mode, delete-vespaset); using-kubernetes-with-vespa converted to a redirect, matching upstream

Full Japanese translation

• All 362 English pages now have a Japanese counterpart under ja/ (mirrored directory tree, including a translated landing page at ja/index)
• Internal links rewritten to /ja/...; code blocks, commands, identifiers, and table markup preserved verbatim
• The four legacy ja/ pages (which translated long-outdated content) were replaced by the full mirrored tree

Bilingual navigation

• docs.json navigation restructured into languages: en (default) and ja, with translated tab/group labels and fully symmetric page trees (309 nav pages per language)

Quality checks

• Every table in both languages is an HTML table (no markdown pipe tables)
• mint broken-links passes with no errors across all 724 pages
• docs.json validated: every referenced page exists; en/ja nav symmetry verified