Add documentation on input types

Docs/SpecialInputElements

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+# Available types for text fields
+
+The listed types here trigger a special input element. Use them in `tagrendering.freeform.type` of your tagrendering to activate them
+
+## string
+
+A basic string
+
+## text
+
+A string, but allows input of longer strings more comfortably (a text area)
+
+## date
+
+A date
+
+## wikidata
+
+A wikidata identifier, e.g. Q42
+
+## int
+
+A number
+
+## nat
+
+A positive number or zero
+
+## pnat
+
+A strict positive number
+
+## direction
+
+A geographical direction, in degrees. 0° is north, 90° is east, ... Will return a value between 0 (incl) and 360 (excl)
+
+## float
+
+A decimal
+
+## pfloat
+
+A positive decimal (incl zero)
+
+## email
+
+An email adress
+
+## url
+
+A url
+
+## phone
+
+A phone number
+
+## opening_hours
+
+Has extra elements to easily input when a POI is opened
+
+## color
+
+Shows a color picker