doc: adds LN documentation

docs/ln.md

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+# LN
+
+## Syntax
+
+```
+LN(numericExpression)
+```
+
+
+## Description
+
+Returns the Natural Logarithm of the _numeric expression given_.
+A natural logarithm is a logarithm in base _e_ (_e_ value can be
+obtained with `EXP(1)`).
+
+Both the argument and the result are of type `Float`.
+
+## Examples
+
+```basic
+PRINT "LN(10) is "; LN(10)
+```
+
+You can compute the logarithm in any `base` dividing `LN(x)` by `LN(base)`:
+
+```basic
+PRINT "LN2(8) is "; LN(8) / LN(2) : REM Base 2 Logarithm
+PRINT "LOG(100) is "; LN(100) / LN(10) : REM Base 10 Logarithm
+```
+
+
+## Remarks
+
+*  This function is 100% Sinclair BASIC Compatible
+*  If the given argument type is not float, it will be [converted](cast.md) to float before operating with it.
+
+## See also
+
+* [EXP](exp.md)