Similar to #12226 but slightly more ambitious.

I think it's be great if we can reorganize the top level page documents because

• The list is currently super long, which requires readers to always scroll down to see the class list

  See the screenshot

  

• The pages are not well organized so it's not clear how pages are related to each other or their purpose, without clicking into them

(It also annoys me that the names are all underscored. I'd like to change it but it'd be a RDoc feature instead)

This PR is for

• Moving current top-level pages into similar groups
• Renaming some of the pages for clarification
• Updating all the related rdoc-ref references in the code

This PR is NOT for

• Rewriting or merge existing documentations' contents
• Cleanup unused page documentations

After

Pages list folded

Pages list with `language` folder expanded

Help needed

• Better name for the language group. Maybe language_features?
• Better names for pages