docs: Clarify app icon instructions, add link to Material Symbols & Icons #35177

Job	Run time
Build and stage / Prepare PDF build image	14s
Build and stage / Building user_manual ePub	29s
Build and stage / Building admin_manual HTML	59s
Build and stage / Building developer_manual ePub	52s
Build and stage / Building developer_manual HTML	1m 19s
Build and stage / Building admin_manual ePub	53s
Build and stage / Building user_manual-en HTML	46s
Build and stage / Building user_manual HTML	4m 34s
Build and stage / Building admin_manual PDF	3m 20s
Build and stage / Building user_manual PDF	5m 40s
Build and stage / Stage and check documentation	22s
Deploy Netlify preview	1m 4s
Check for broken links	17s
build-deploy-summary	8s
	20m 57s

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