[Doc] nix install option

[dannypsnl]

Explanation

This PR add an install option via nix, so we can get more reports about this installation way.

Related issue

resolve #248

What type of PR is this

/kind documentation

Proposed Changes

Add install option

[juntao]

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

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Commit 1c5f08d290e3beecee251dbd4057743e37f6c105

1. The patch introduces a new installation method for WasmEdge using Nix, which is beneficial for users of the nix/nixos system.
2. This new method allows users to install WasmEdge with a single command: nix profile install github:WasmEdge/WasmEdge.
3. The patch does not affect any other installation methods or content in the documentation.
4. No changes were made to the Windows installation instructions.
5. The patch closes issue [DOCS] install via nix flake #248, but no specific details about this issue are provided in the patch description.