docs: add demos section - #9

Changed pages include:

• index.html - view
• 200.html - view
• 404.html - view
• awesome/index.html - view
• demos/index.html - view
• zh/index.html - view
• concepts/background/index.html - view
• concepts/content/index.html - view
• concepts/manifest/index.html - view
• guide/introduction/index.html - view

There are more changes not included in the list