Show module docstring for namespace indexes

[scarf005]

• [x] I have read CONTRIBUTING.md.
• [x] I have checked that there is no existing PR/issue about my proposal.

Summary

Before	After

Motivation

Hoogle	Idris2

search result on hoogle shows module summary, which is very helpful for beginners.

The proposal

As shown in summary, provide brief summary for each modules.

Technical implementation

1. use module docstring

||| Types and functions for reasoning about time.
module System.Clock

will output Types and functions for reasoning about time.

2. add it to resulting html.
3. truncate if it's too long.
4. use flexbox and justify-content: space-around to make it pretty.