Accessibility: Privacy: Accessibility improvements for the Privacy Policy Guide page.

Improves accessibility of the "Copy this section" button and "Return to Top" link:
- uses `setTimeout()` and `clearTimeout()` to properly handle the "Copied!" text
- simplifies the button text by removing the redundant visually hidden text
- fixes the mismatching visual and DOM order of the Copy button and the "Return to Top" link 
- improves the "Return to Top" links by providing real page fragment identifiers, when possible
- hides the "Return to Top" up arrow from assistive technologies
- minor coding standards

Props afercia, garrett-eclipse.
See #48463, #50322.
Fixes #50335.

Built from https://develop.svn.wordpress.org/trunk@48234


git-svn-id: https://core.svn.wordpress.org/trunk@48003 1a063a9b-81f0-0310-95a4-ce76da25c4cd