Cheat sheet for annotations

Present a compact cheat sheet for decorators relevant for code generation. Here's a list of all decorators. Irrelevant ones to be removed:

• @@deprecated
• @@warning
• @as only used for runtime representation, except for default arguments @as("someString") _
• @bs.send.pipe
• @dead
• @deprecated
• @deriving About runtime representation
• @doesNotRaise
• @genType
• @get
• @get_index
• @ignore
• @inline
• @int not relevant: about runtime representation
• @live
• @meth About runtime representation
• @module
• @new
• @obj
• @raises
• @react.component
• @return This mixes runtime representation and externals: a post-processing for results
• @scope
• @send
• @set
• @set_index
• @string not relevant: about runtime representation
• @this Requires annotations on both external and uses
• @unboxed About runtime representation
• @uncurry This guarantees that the curried function is treated as if it were uncurried
• @unwrap Unwraps one level of unary polymorphci variants (e.g. [#Int(int) | #Str(string)]). Should probably be a language-level feature either<(int, string)>.
• @val
• [?] @variadic Should this be a language-level feature?

A few examples covered:

// @val
// external setTimeout: (unit => unit, int) => timeoutID = "setTimeout"
external setTimeout: (unit => unit, int) => timeoutID = "setTimeout($1, $2)"

// @get external getName: window => string = "name"
external getName: window => string = "$1.name"

// @set external setName: (window, string) => unit = "name"
external setName: (window, string) => unit = "$1.name = $2"

// @get_index external get: (t, string) => int = ""
external get: (t, string) => int = "$1[$2]"

// @set_index external set: (t, string, int) => unit = ""
external set: (t, string, int) => unit = "$1[$2] = $3"

// @module("path")
// external dirname: string => string = "dirname"
external dirname: string => string = "import(path).dirname($1)"

// @new external create: unit => t = "Date"
external create: unit => t = "new Date()"

// @send external getElementById: (document, string) => Dom.element = "getElementById"
external getElementById: (document, string) => Dom.element = "$1.getElementById($2)"

// @module("library-x")
// @val external doStuff: (@as(json`{format:"utf8", includeStuff: false}`) _, string) => string = "doStuff"
external doStuff: string => string = "import(library-x).doStuff({format: 'utf8', includeStuff: false}, $1)"

// @val external doSomething: (@ignore 'a, 'a) => unit = "doSomething"
external doSomething: ('a, 'a) => unit = "doSomething($2)"

// @obj
// external action: (~name: string, unit) => _ = ""
external action: (~name: string, unit) => _ = "{name:$1}"

// @scope("Math") @val
// external floor: float => int = "floor"
external floor: float => int = "Math.floor($1)"

Start with the decorator list and ReScript external examples in this issue, then locate the existing annotations documentation in the site. Create a compact cheat sheet covering the relevant code-generation decorators and clearly separating or removing the irrelevant ones; done means the listed examples and remaining annotations are represented accurately.