Adding a book about Markdown

Hello! I would like to contribute a free programming book to the Russian language list (books/free-programming-books-ru.md).

I am the author of this book, and I confirm that it is completely free to read, download, and modify. It is distributed under the MIT License (see the LICENSE file in the repository). The book is hosted on GitHub Releases, which is an authoritative source and fully complies with the repository's guidelines (no restricted file-hosting platforms are used).

Proposed addition:

• Документация по языку текстовой разметки — Markdown - Изергиль Ван Дер Вельде (PDF) (MIT)

Book Details: Title: Документация по языку текстовой разметки — Markdown
Author: Изергиль Ван Дер Вельде (Izergil Van Der Velde)
Organization: Cultus Tenebrae (GitHub)
Language: Russian
Format: PDF
Source: GitHub Repository
Status: Released (with ongoing updates)

Description:
This book covers all the major standards and topics for the Markdown text markup language. It includes numerous examples of working syntax and references to original authoritative literature, making it a comprehensive guide for both beginners and experienced users.

Thank you for maintaining this excellent resource. Please let me know if any adjustments are needed to fully comply with the contribution guidelines.