docs: add Simplified Chinese README translation

[whtis]

What

• Add README.zh-CN.md, a Simplified Chinese translation that mirrors README.md 1:1 in structure (sections, tables, <details> blocks, image paths, anchor links).
• Add a centered language-switcher line near the top of README.md linking to the Chinese version.

Why

The existing audience for RustNet skews internationally, and PR #109 already lays the groundwork for in-app Chinese localization via rust-i18n. Adding a Chinese README extends the same discoverability to first-time Chinese-speaking visitors landing on the GitHub page, who currently see only English. The two efforts are complementary: PR #109 covers CLI/TUI strings, this PR covers project discovery and onboarding.

Translation choices:

• Kept all technical terms in English where that is the convention in Chinese systems writing (TCP/UDP/QUIC, eBPF, libpcap, ratatui, MaxMind GeoLite2, Landlock, Seatbelt, SNI, GeoIP, DPI, PKTAP, sysfs, getifaddrs, etc.).
• Preserved every relative link, anchor, and image path so the docs cross-reference network still works.
• Used full-width Chinese punctuation in prose, half-width parentheses/colons when wrapping pure English or code identifiers, matching the prevailing style of high-quality Chinese tech READMEs.

How tested

• git diff --stat: 2 files changed, 324 insertions(+), 0 deletions(-).
• Side-by-side compared each section against README.md to confirm structural parity (same headings, same code blocks, same number of bullets, same <details> toggles, same table rows).
• Verified every relative link target (USAGE.md#pcap-export, ARCHITECTURE.md#dependencies, INSTALL.md#ebpf-unavailable-despite-capabilities-being-set, etc.) exists in the repo so anchors don't break in the translated copy.
• Rendered locally; no broken markdown.

Happy to iterate on any phrasing if you'd like a particular term changed — and feel free to close this if you'd rather wait until the i18n work in #109 lands so the README and CLI translations can land together.

Drafted with AI assistance; verified by author before publishing.

[domcyrus]

Hi @whtis, thanks for your contribution. This looks good to me. If you want to remove the draft label I'll merge it.

[whtis]

Done — marked as ready for review. Thanks @domcyrus