docs: migrate nio12l NPU page and update g2d guide for new images

[Ronin-1124]

Description of changes

nio12l: NPU environment setup migrated to dedicated page

• Extracted NPU setup content from ubuntu-user-guide.md into
  npu-usage/env-setup.md
• Merged two installation flows: apt-based firmware/runtime (from user guide)

• snap-based Neuron SDK (original env-setup)

• Both pages now cross-reference each other

cubie: G2D usage guide updated for new images

• New images ship with G2D kernel module enabled by default
• Changed :::info → :::tip with a link to skip ahead to driver
  verification
• Wrapped legacy kernel install commands in <details> fold for older images
• Applied to both zh and en versions of the shared partial

Agent-doc checklist

• I ran scripts/agent-doc-lint.sh on changed doc files.
• Changed page docs are not empty, include front matter, and have an H1
  heading.
• Code fences in changed docs include language labels.
• I removed TODO/TBD/XXX placeholders from non-template docs.
• I reviewed whether docs/ and i18n/en/... need to be synced.
• I did not introduce new docs/i18n path drift, or I updated
  .github/agent-doc-drift-baseline.md intentionally.
• I did not introduce new translation placeholders, or I updated
  .github/agent-doc-translation-baseline.md and
  .github/agent-doc-translation-backlog.md intentionally.

Validation notes

• pre-commit: passed
• additional checks: zh/en content synced for both nio12l and cubie changes