Syntax lookup for various decorators

[kevanstannard]

Some suggestions for #162

I'm currently getting familiar with ReScript and related terminology so I'm not attached to any content in this PR, and I'd be grateful to receive guidance on improvements and corrections (including nitpicks).

Feel free to edit if you prefer.

Some of this content is derived from the old docs, otherwise it's a best effort based on my current understandings.

Thanks

[kevanstannard]

@ryyppy thanks for your feedback, was very helpful, particularly clarifying the goals of the syntax lookup vs core documentation. I've cleaned up all of these pages and tried to standardise; simple description, one example, plus links to references. Is that closer to what you had in mind?

Any further feedback is welcome, and feel free to modify as you like.

Also, I wasn't sure how best to indicate the status of @deriving, but take a look and see what you think.

There are a couple of remaining decorators not included in this PR that I wasn't sure about, so I'll leave them out for now.

Otherwise, ready for another review 👍

[ryyppy]

yeah way cleaner and uniform now. The references also really make the difference!

Will merge this later today, then do some further merges into master, and will post an update in our core issue and tick all the decorators that were added here. We will tackle the rest later on if necessary!