I2I: <amp-sidebar> v0.2

[leafsy]

Summary

Upgrade <amp-sidebar> to version 0.2 and introduce a new <amp-nested-menu> component for rendering nested drill down menus.

Design document

https://docs.google.com/document/d/1FNduZqgOU8yl3j6_b-VpTvp9cmtbCfNQ25Vm8w-BXQc/edit?usp=sharing

Motivation

Improve user experience and feature parity with existing sidebar component implementations, namely:

• Provide first class support for nested drill down menus (e.g. amazon.com);
• Support animations of the sidebar and document body for open and close;
• Move sidebar mask inside the component itself (Make <amp-sidebar> mask element compatible with Story #24827);
• Ensure that sidebar is compliant with accessibility standards and supports accessibility best practices.

Additional context

[nainar]

Accessibility considerations: https://docs.google.com/document/d/12_F6DpT_KGHcONEhW04P_uPYu8V_q05JYQ5PmVF-LEE/edit