docs: Add comprehensive Docusaurus documentation site

[teyouale]

https://teyouale.github.io/fp-go/docs/concepts/monads/

[CarstenLeue]

I have two main points:

• the build output should go to the gh-pages branch
• the examples should NOT be part of the markdown files, but rather go examples pulled into the markdown at build time

[CarstenLeue]

should use the latest LTS

[CarstenLeue]

Where does this deploy to?
In a github pages deployment I would expect this to be deployed to the branch that actually serves up github pages.

[CarstenLeue]

This code should not live in this MD file, because nothing validates that this is actually compilable code.

Instead all code examples should reference an _test.go file and in that file an ExampleXXX function. That way all examples are executed at compile time and we are sure they compile.

When the docusaurus site gets build the examples need to be inserted into the docusaurus markdown.

This can e.g. be done via the https://github.com/campoy/embedmd tool (running as a pre-processor over the docusaurus files) or via a remark plugin.

[CarstenLeue]

Should be

func processUser() readerioresult.ReaderIOResult[string, *User] {
    return function.Flow3(
        fetchUser,
        ioresult.Chain(validateUser),
        ioresult.Chain(saveUser),
    )
}

[CarstenLeue]

The example should be

func processUser() readerresult.ReaderResult[string, User] {
    return function.Flow4(
        fetchUser,
        result.Chain(validateUser),
        result.Chain(enrichUser),
        result.Chain(saveUser),
    )
}