docs(nav): drop dead case entry and repoint Supervisor to the renamed page - #23

Fixes #3

What changed

Two entries under 最佳实践案例 in mkdocs.yml pointed at files that do not exist:

On the second one: there is no supervisor-mode.md, but user/cases/total-agent-mode.md
(案例七：总控智能体模式——多角色自动编排) was the only case page missing from nav, and its
content is exactly the supervisor story — "自动拆解任务、分配不同 AI 角色去干、监督进度、出错
自动调整方案……它就像是你的技术总监". So the entry looks like a rename that nav never followed.
I relabelled it to match the actual page title rather than leaving the stale English label.

If you actually intended a separate Supervisor page that still needs to be written, say so and
I'll restore the entry instead.

How this was checked

mkdocs build --strict before the change:

WARNING -  A reference to 'user/cases/claude-code-continuous.md' is included in the 'nav' configuration, which is not found in the documentation files.
WARNING -  A reference to 'user/cases/supervisor-mode.md' is included in the 'nav' configuration, which is not found in the documentation files.
INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
  - user\cases\total-agent-mode.md

After the change both WARNINGs are gone and total-agent-mode.md no longer appears in the
not-in-nav list.

I also walked every nav entry (72 → 71 after the removal) and every link in docs/index.md
against the filesystem: 0 remaining nav entries resolve to a missing file, and 0 of the 27
relative links in index.md are broken.

Not changed

• mkdocs build --strict still fails overall on pre-existing anchor warnings (36 of them,
  mostly Chinese anchor links under user/guides/configuration/smart-employees.md) plus
  index.md / contribute/*.md links that intentionally point to repo-root files
  (../CONTRIBUTING.md, ../SUPPORT.md, …). Those are outside this issue's scope and I did not
  want to bundle a large link rewrite into one PR.
• Five pages are still not in nav: user/explanation/asset-center.md,
  user/guides/configuration/asset-center.md, .../resource-packs.md,
  .../install-own-project-as-extension.md, assets/sponsors/README.md. Left alone since I
  can't tell whether they're intentionally unlisted — happy to add them in a follow-up.

AI involvement

Drafted with AI assistance. The nav/link audit and the mkdocs build --strict before-and-after
were verified directly against a local checkout.