Update develop_plugin_c.md

[peter-jerry-ye]

Explanation

Make the code example for developing plugins in C complete. Now one can copy all the code snippets and expect it to work.

Related issue

Closes Wasmedge/Wasmedge#3122

What type of PR is this

/kind documentation

Proposed Changes

Copied the missing part of testplugin.c into the document

[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 4ee30642d3777492adc54ff3ad6da88131187bfc

Key Changes:

• Added a new function WasmEdge_Plugin_GetDescriptor to export the plug-in descriptor.

Potential Problems:

1. The newly added function WasmEdge_Plugin_GetDescriptor does not have any input parameters or return type specified. It's unclear if this aligns with the existing coding standards and conventions, as normally functions have explicit input parameters and return types defined.
2. There is no description provided for the function WasmEdge_Plugin_GetDescriptor, which may make it difficult for other developers to understand its purpose and proper usage.
3. The patch does not include any test cases or updates to the existing tests for the changes made in the code. It's important to ensure that new functionality is properly tested to maintain code quality.
4. It would be beneficial to include comments explaining the purpose of exporting the plug-in descriptor and how it fits into the overall functionality of the plugin. This adds clarity for future developers who may work on this codebase.

[alabulei1]

Hi @peter-jerry-ye

Thanks for the PR. Could you please fix the DCO issues? Thanks.