update apple-touch-icon guidance

[paulirish]

What's changed, or what was fixed?

• Updated the guidance on apple-touch-icon
• removed [sizes] examples. clearer reccs on precomposed, the image size, etc.

See https://webhint.io/docs/user-guide/hints/hint-apple-touch-icons/ to verify these recommendations.

Target Live Date: asap

• This has been reviewed and approved by @kaycebasques
• I have run npm test locally and all tests pass.
• I have added the appropriate type-something label.
• I've staged the site and manually verified that my content displays correctly.

CC: @petele

[paulirish]

A brand new PWA audit in lighthouse links to this page.. and a number of PWAs are failing on it.. So i'm eager to get these changes live. :)

[WebFundBot]

Whoops!

There were 5 warnings that will prevent this PR from being merged. Please take a look, and either fix, or provide a justification for why they can't be fixed.

WARNINGS
src/content/en/fundamentals/design-and-ux/browser-customization/index.md#L19 - Common typo found: 'webpage' Should it be 'web page'? See: https://developers.google.com/style/word-list#web_page
src/content/en/fundamentals/design-and-ux/browser-customization/index.md#L14 - Line must be at most 100 characters
src/content/en/fundamentals/design-and-ux/browser-customization/index.md#L19 - Line must be at most 100 characters
src/content/en/fundamentals/design-and-ux/browser-customization/index.md#L137 - Line must be at most 100 characters
src/content/en/fundamentals/design-and-ux/browser-customization/index.md#L143 - Line must be at most 100 characters

[kaycebasques]

Let's have a looksie

[WebFundBot]

🎉 This has been pushed live to https://developers.google.com/web/