[Docs] Add entries about `Variant` and `Optional` to Mojo manual

[gryznar]

Where is the problem?

https://docs.modular.com/mojo/manual/

What can we do better?

Manual is missing mention about Variant and Optional from collection. This features are very powerful and useful, but badly exposed. For most users manual is main entry point and they are missing them, because they are placed somewhere deeper in documentation

Anything else?

No response

[gryznar]

@Brian-M-J thanks! Fixed

[arthurevans]

This (or at least Optional) probably belongs in a section on stdlib collection types.