Uncommented the Bootstrap section in the index.html file, which enables the Bootstrap section on the website. Also, added substantial content to the newly created md/html/bootstrap.md file to cover Bootstrap related materials in detail. Corrected the CSS title format in the md/html/css.md file for proper markdown rendering.

The lang attribute does not default to English. It defaults to an unknown, which is an accessibility issue.

Readme says the default is 'history: false', so it may be surprising
for users - it's easy to miss the contradicting setting in index.html