Docs site: language switch, Agent Skills nav, and CN intro deck

[RogerLiu312]

Brings only the docs-site/ changes from develop onto main. None of PR #15's
src/sflow code, tests, docs/user pages or examples are included — git diff main..HEAD -- ':!docs-site'
is empty.

Cherry-picked b15051c + 6db2b64 (they applied cleanly), plus one new redaction commit.

What lands

• Chinese intro deck (sflow_intro_zh.html) and an EN/CN language switch on both decks.
  Language is carried in the query string (/?lang=zh) rather than swapped in place, so the
  Chinese deck is shareable, bookmarkable and survives a reload. The current slide is carried
  across the switch via the URL hash.
• Agent Skills navbar entry, promoted out of the Docs sidebar into its own top-level
  agents sidebar so it is a peer of Docs. The entry is version-aware; v0.1.0 predates the
  skills feature and correctly gets no agents sidebar.
• Perspective Shift slide: Infra platform now sits under Software, which matches how the
  stack actually layers. Animation timing and the colour ramp stay bound to position, so the
  bottom-up build wave and the green-to-cyan gradient are unchanged.
• CN deck logo fix: a stale ../../../ image path left over from when the file lived in
  docs/developer/dev-notes/.
• Two previously legacy-hidden slides restored, taking the deck from 16 to 18 slides.

Redaction

The restored "Structured Error Analysis" slide had never been public and carried a real
allocation map — two internal Slurm hostnames and the frontend's private address. Replaced
with neutral placeholders in both decks. The slide illustrates how a run is diagnosed, so the
exact host names and IP carry no explanatory value.

Verification

• 19/19 docs-site tests pass.
• Full versioned build succeeds at baseUrl=/nv-sflow/ (the one broken-anchor warning on
  docs/v0.1.0/sflow_srd predates this branch).
• Checked in a headless browser: language round-trips / ↔ /?lang=zh and holds slide
  position; Agent Skills activates its own sidebar and v0.1.0 does not 404; the redacted
  slide renders with no overflow.