Theming: Add documentation on themed icons

[juliusknorr]

Add documentation for imagemagick dependencies on nextcloud/server#840

[MorrisJobke]

Ping @juliushaertl ;)

[juliusknorr]

@MorrisJobke done in #233 😉

[MorrisJobke]

@MorrisJobke done in #233 😉

but it is #248 😉

[juliusknorr]

🙈 to much numbers, but you've found it 😉