Summary:
Rewrote the Icons page with the following goals in mind:
- Offer concrete guidelines to help people design new icons
- Accurately describe existing icon conventions
- Document the dynamic color changing mechanism for monochrome icons
- Replace outdated images
- Be more focused and less rambling/stream-of-consciousness-ey

Also adjusted the Emblems page to point to the new icons page, where applicable.

Test Plan:
Your best bet is probably to compile it and read the pages yourself (see
https://hig.kde.org/resources/contribute.html#getting-started)

Reviewers: fabianr, #vdg, #kde_human_interface_guidelines, ndavis

Reviewed By: #vdg, ndavis

Subscribers: trickyricky26, abetts, ndavis

Tags: #kde_human_interface_guidelines

Differential Revision: https://phabricator.kde.org/D16848