docs: add package cache usage guide (en/zh)

[Goalina]

What

Adds a new documentation page explaining how to use the in-cluster package cache on Ascend NPU runners, in both English and Chinese:

• docs/user-manual-cache-en.md
• docs/user-manual-cache-zh.md

Repo.md already shows a per-repo PyPI/APT cache audit table, but there was no page describing what the cache is or how to enable it. This fills that gap.

Contents

Covers all package managers the cache proxy supports (nginx-pypi-cache):

Manager	Port	Endpoint
pip / PyPI	80	/pypi/simple
PyTorch wheels	80	/whl/<variant>
APT (Ubuntu / Debian)	8081	:8081
rustup	8082	:8082
yum / dnf (openEuler)	8083	:8083

Each section gives copy-paste setup commands, a verification step (X-*-Cache headers), plus a full end-to-end workflow example and FAQ. Notes that the cache is opt-in, cluster-internal (only reachable from inside the job container), and configured manually per package manager.

Also

• Wires both pages into the mkdocs.yml nav (after the Buildkite guides).
• Adds a link from the Getting Started page.

Commands are sourced directly from the nginx-pypi-cache deployment config and its README.