Implemented linq extensions

[DarthPedro]

Added

• Added full documentation set for this library, including introduction, getting started, and multiple Api reference documents.
• LINQ query syntax support for Optional<T> and Result<T>. New OptionalLinqExtensions and ResultLinqExtensions provide Select, SelectMany, and Where, enabling from ... select ... comprehension syntax. These extensions live in the main D20Tek.Functional namespace so query syntax works with the same using that imports the core types. Result<T>.Where takes an explicit Error argument so a rejected value never produces a hidden or invented error.
• Added unit tests for new extension methods.