Add piper backend

[PeterD1524]

Explanation

I am working on LFX Workspace: Support piper as a new backend of the WASI-NN WasmEdge plugin #3483.
This PR adds a tutorial to build the piper plugin.

Related issue

WasmEdge/WasmEdge#3483

What type of PR is this

/kind documentation

[alabulei1]

Hello, I am a code review bot on flows.network. Here are my reviews of code commits in this PR.

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Commit 38fa81019202f4ec77eeb1377d9ab17036ee057e

Key Changes:

1. Addition of documentation on building WasmEdge with WASI-NN Piper Backend in the wasi_nn.md file under the docs/contribute/source/plugin/ directory.
2. Instructions provided for building and installing WasmEdge from source with the Piper backend for WASI-NN.
3. Information on setting the WASMEDGE_PLUGIN_PATH environment variable if the wasmedge CLI tool cannot find the WASI-NN plug-in.

Potential Problems:

1. Security Concerns: The patch includes an email address in the Signed-off-by section, which may expose personal information. It's recommended to use a username or display name instead.
2. Consistency in Path: The patch mentions different paths for the executable wasmedge runtime and the WASI-NN with Piper backend plug-in installation. It would be good to ensure consistency in paths for better clarity.
3. Error Handling: While the patch provides guidance on setting the WASMEDGE_PLUGIN_PATH environment variable if needed, it might be beneficial to include troubleshooting steps or potential reasons why the CLI tool cannot find the plug-in in the first place.

[mhmohona]

Thank you for updating doc! ✨

[hydai]

Hi @PeterD1524
Since you updated the same file in another PR, you will need to update this to fix the conflicts. Thanks.