fix(docs): replace HTML heading tags with standard Markdown headings and fix hierarchy

[not-knope]

Description

This PR fixes Markdown heading rendering and outline generation issues reported in #68.

Changes Made:

• Replaced raw HTML <h1 align=center>\ and <h3 align=center>\ tags in \README.md\ with standard Markdown #\ / ##\ headings wrapped in <div align=center>. This allows GitHub, Hugging Face, and Markdown parsers to register them in document outlines/table of contents while preserving visual centering.
• Fixed heading level skipping under ### vLLM\ (##### Docker Images\ -> #### Docker Images).
• Fixed section heading level in \CONTRIBUTING.md\ (# Chinese version\ -> ## Chinese Version).
• Added an automated test script (\ ests/test_markdown_headings.py) to validate Markdown heading syntax and hierarchy.

Fixes #68