feat(picker): page-up/dn preview scroll, help improvements

[minusfive]

Description

This PR:

• Adds <PageUp> and <PageDown> maps to scroll preview (n, i).
• Adds <c-/> map to toggle help (n, i), similar to telescope and essentially using the same ? key but from i mode.
• Adds descriptions to all input window key maps and better organizes them (code + display).
• Adds icons to some keymap descriptions.
• Tweaks default help window column widths and removed separator to maximize doc space (I think highlights + alignment are enough to visually separate them, no need for extra chars).

Related Issue(s)

N/A

Screenshots