feat(ci): add Android (aarch64) cross-compilation for Termux users

[houko]

Summary

Closes #886

• Add aarch64-linux-android target to Cross.toml using the official cross-rs Android image
• Add cli_android job to release-shell.yml that cross-compiles the CLI binary for Android ARM64 and uploads it to GitHub Releases
• Add Android/Termux setup documentation (English + Chinese) covering installation, configuration, usage, and troubleshooting
• Register the new docs page in the navigation sidebar

Details

The goal is to let Termux users on Android download a pre-built binary from releases without needing to compile from source. The CI job uses cross (cross-rs/cross) with the ghcr.io/cross-rs/aarch64-linux-android:main image and produces a librefang-aarch64-linux-android.tar.gz artifact.

Test plan

• Verify CI workflow syntax is valid (GitHub Actions will validate on PR)
• After merge and tag push, confirm the cli_android job runs and uploads the artifact
• Download the artifact on an Android/Termux device and verify ./librefang --version works

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