DOC-3221: Refine language and structure in CONTRIBUTING.md for clarity and consistency.

[kemister85]

Ticket: DOC-3221

Changes:

• All references to TinyMCE are accurate and reflect the current product version and supported tools (e.g., Antora, AsciiDoc).
• All personal and second-person pronouns (e.g., “we,” “you,” “our,” “your,” “I”) are removed or reworded to meet Tiny style standards.
• Language is consistent with the TinyMCE documentation style guide: Formal tone, Active voice, uses en-US spelling and avoids contractions, slang, jargon, and metaphors.
• Markdown or HTML syntax for links, code blocks, and live demos is updated and correctly formatted using AsciiDoc conventions.
• Section structure is clear and consistent, using appropriate heading levels.

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• Visual check for correct document render.

Review:

• Documentation Team Lead has reviewed