[spec] Restore *ArrayInitializer*

[ntrel]

It was merged with ArrayLiteral in #3215 - sorry.

Fixes Bugzilla 24634 - Parse error initializing array from expression with StructInitializer.

Add examples.
Move index initializer paragraph to arrays.dd, add subheading.
declaration.dd: Add initializers subheading, move paragraph about default init above it.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24634	normal	Parse error initializing array from expression with StructInitializer