Add missing javadoc

[xgouchet]

This PR adds Javadoc/KDoc documentation on public classes, data classes, functions and enums to make it easier to use from IDEs.

Fixes #2016

Specifically:

• JavalinConfig + all classes in the io.javalin.config package
• Bundled plugin configs
• EventManager
• RequestLogger#handle, Handler#handle, ExceptionHandler#handle
• FileRenderer#render
• Cookie 
• ContentType  and HandlerType enums (among others)

Note

Most documentation text was copy-pasted from the MDN Docs pages, and the Javalin documentation pages.

[zugazagoitia]

LGTM!
@tipsy rebase your #2006 plugin PR if you merge this to update the docs in the changed classes

[dzikoysk]

Good job @xgouchet :) A few things that I'd address here:

1. Some comments don't really provide useful information. I'm mainly talking about method parameters like e.g. @param contentType the content type to add on fun add(contentType: ContentType) { method. I think it's fine to just omit it & have just one simple one-liner comment about the function as a whole.
2. Highlight that methods have default values for parameters. It might be a good idea to describe the default behavior in the docs, especially when @JvmOverloads is used.
3. I'd reduce unnecessary spacing - empty lines & use one-liners whenever possible (Context.kt is a good visual reference here)

[xgouchet]

I followed your suggestion @dzikoysk , and also added missing documentation on the HttpStatus enum 👍

[tipsy]

This looks really good @xgouchet ! Are you ready for merge?

[xgouchet]

Yes all good for me

[dzikoysk]

LGTM, I think these docs will help our users. It's not only describing particular functions, but also brings a general context around them :)

[tipsy]

Thank you very much @xgouchet !