Raspberry pi installation instructions are outdated

[stephen-lev]

https://github.com/opensensor/lightNVR/blob/main/docs/INSTALLATION.md#2-build-and-install

The following command gives an error:

# Build with Raspberry Pi optimizations
./scripts/build.sh --release --platform=raspberry-pi

Unknown option: --platform=raspberry-pi
Use --help for usage information

The --help option doesn't mention platform option or any similar.

[matteius]

• Removed the non-existent --platform=raspberry-pi flag — the correct command is just ./scripts/build.sh --release (CMake auto-detects ARM library paths)
• Updated the intro to mention pre-built .deb packages are available for arm64/armhf on both trixie and sid (which we just added in the previous commit)