[Epic] Unified docs for Splash Screen feature

[lukeblevins]

Task items

unoplatform/uno.toolkit.ui

• ExtendedSplashScreen is not documented uno.toolkit.ui#574

unoplatform/uno

• docs: Align existing splash screen info, minor docs updates #14868
• docs: Add wasm manifest content to splash screen feature docs #15244
• Overview page for splash screen feature #15431
• Splash Screen for GTK #13338

unoplatform/uno.wasm.bootstrap

Alignment between WebAssembly section in general docs and similar topic under the bootstraper feature

• Determine action needed for existing bootstrapper page

CC: @kazo0, @agneszitte, @jeromelaban

[lukeblevins]

This work is the first component to a larger effort around aligning + improving our resources for setup/customization of splash screens:

• Positioning existing content as a manual option post-Resizetizer while still preserving docs relevant to that scenario
• Documenting the toolkit's ExtendedSplashScreen as the recommended solution for GTK
• Modifying bootstrapper repo page Splash screen customization to instead be a reference for manifest file properties--preventing it from being mistaken by everyday consumers of splash screen
• Expanding what the toolkit repo has documented for ExtendedSplashScreen

...

I think what's above better explains the work being tracked here (for #14868)