added nan/na doc file

Author: Jolanrensen

docs/StardustDocs/d.tree

@@ -35,6 +35,7 @@
         <toc-element topic="DataColumn.md"/>
         <toc-element topic="DataRow.md"/>
     </toc-element>
+    <toc-element topic="NanAndNa.md"/>
     <toc-element topic="operations.md"/>
     <toc-element toc-title="Operations">
         <toc-element topic="create.md">

docs/StardustDocs/topics/NanAndNa.md

@@ -0,0 +1,25 @@
+[//]: # (title: NaN and NA)
+
+Using the Kotlin DataFrame library, you might come across the terms `NaN` and `NA`. 
+This page explains what they mean and how to work with them.
+
+## NaN
+
+`Float` or `Double` values can be represented as `NaN`,
+in cases where a mathematical operation is undefined, such as for dividing by zero. The
+result of such an operation can only be described as "**N**ot **a** **N**umber".
+
+This is different from `null`, which means that a value is missing and, in Kotlin, can only occur
+for `Float?` and `Double?` types.
+
+
+You can use [fillNaNs](fill.md#fillnans) to replace `NaNs` in certain columns with a given value or expression
+or [dropNaNs](drop.md#dropnans) to drop rows with `NaNs` in them.
+
+## NA
+
+`NA` in Dataframe can be seen as: [`NaN`](#nan) or `null`. Which is another way to say that the value
+is "**N**ot **A**vailable".
+
+You can use [fillNA](fill.md#fillna) to replace `NAs` in certain columns with a given value or expression
+or [dropNA](drop.md#dropna) to drop rows with `NAs` in them.