Docs: Use a 180x180 apple-touch-icon.png.

Refs:
https://github.com/h5bp/html5-boilerplate/commit/1c4fcf32142e888946db579df0efbb52542e0ed1
https://github.com/h5bp/html5-boilerplate/commit/345925f04384b59929d8d39ba428cef548921408

[ci skip]
new icon
Merge pull request #14748 from twbs/apple-touch-icon

Docs: Use a 180x180 apple-touch-icon-precomposed.png.