[feature] Include overview sections

[nelson6e65]

Scopes

• Output HTML
• Output Markdown

Describe the solution you'd like

• Include a general overview as the main page and move help description to an optional page.
• Include an overview section in each API page (classes.md, etc), containing an index of elements.
• Include an overview section of each structure (class, interface, trait) containing a list/table of its members.