Ported/Converted NativeCoreAPIReference_en_r13.pdf file from PDF to Markdown

[thynetruly]

Hello! I emailed the Live2D team last year about my comment translation script since I had successfully used it to translate the comments throughout the Live2D Cubism SDK for Native files into English for my team's internal use. Hope this can help the Live2D developer community by increasing accessibility to the API documentation! Please let me know if there are any changes, proofreading, or editing needed since I retained the original file's grammar. I tried very hard to treat this as a reference document with all of the information from the source PDF file properly reflected. I'm also willing to port the Japanese PDF, so please let me know if you need that.

[Copilot]

Pull Request Overview

This PR converts the Live2D Cubism Core API Reference from PDF format to Markdown, making the documentation more accessible to developers. The conversion includes all content from the original PDF with proper formatting, code snippets, and internal navigation links.

• Converted the entire NativeCoreAPIReference_en_r13.pdf to Markdown format
• Maintained all original content including changelog, API descriptions, and code examples
• Added proper Markdown formatting for tables, code blocks, and navigation links

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[wada-at-live2d-com]

@thynetruly

Thank you for submitting the pull request!
Sorry for the late reply.

We have thoroughly discussed the migration to Markdown internally.
While ideally, documentation should be written in Markdown and managed on GitHub, the Cubism Core API Reference is a bit special.
The main reason it is provided as a PDF is so that users who do not have access to GitHub can still view it directly as a file. Since there are quite a few such users, we believe we cannot discontinue managing the PDF version.
Additionally, maintaining both Markdown and PDF versions would significantly increase our maintenance costs.

Therefore, we regret to inform you that we are unable to accept this pull request.

Thanks.