Improve organization and add missing examples in API documentation

[piotrwitek]

Table of contents needs improvements in organization for better user experience, also we are missing some built-in examples. Here is a rough plan of needed improvements:

• Improve api list organization and grouping (operations on object keys)
• Add missing built-in types (Record, Parameters, ConstructorParameters)
• Add missing use-case examples to standard-lib API
• Add Optional type alias for Partial

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[geopic]

Hi @piotrwitek, I can work on this issue for you if you'd like me to.