Add Chinese RFC 2217 serial-over-frp deployment guide

Split the manual into a README index plus docs/ sections covering
hardware selection, Linux and Windows serial servers, shared frpc
configuration via env templating, client testing, security, and
troubleshooting. Store real frpc credentials in an ignored .env and
track images with Git LFS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,5 @@
+# 本地真实凭据，切勿提交
+.env
+
+# macOS 元数据
+.DS_Store