docs: add Popover documentation and live examples

[web-padawan]

Added Popover documentation and live examples based on the document.

[web-padawan]

Vale complains about some words e.g. mouseout, listbox, autofocus.

[rolfsmeds]

Dropdown field example

• Missing aria-label
• Doesn't open on focus
• Closes on selections except on to-field?

User Menu

• Doesn't open on kbd-click

[web-padawan]

Doesn't open on focus

Looks like we have a problem with focus trigger combined with nested components like vaadin-select: opening the vaadin-select-overlay makes focus leave vaadin-popover-overlay and then the popover closes.

Reported vaadin/web-components#7652 and updated the description to only use "click" for now.

[web-padawan]

User Menu
Doesn't open on kbd-click

This is problematic as we don't have "keyboard click" logic vaadin-avatar used here. Workarounded this for now by wrapping vaadin-avatar in vaadin-button with theme="icon tertiary-inline" and using button as a target.

[web-padawan]

Rebased and updated the "drop-down field" example to also open on focus in addition to click.