restructure add.md to enable navigation

Author: koperagen

docs/StardustDocs/topics/add.md

@@ -4,7 +4,7 @@
 
 Returns `DataFrame` which contains all columns from original `DataFrame` followed by newly added columns. Original `DataFrame` is not modified.
 
-**Create new column and add it to `DataFrame`:**
+## Create new column and add it to `DataFrame`
 
 ```text
 add(columnName: String) { rowExpression }
@@ -55,7 +55,7 @@ df.add("fibonacci") {
 
 <!---END-->
 
-**Create and add several columns to `DataFrame`:**
+## Create and add several columns to `DataFrame`
 
 ```kotlin
 add { 
@@ -130,41 +130,41 @@ df.add {
 </tab></tabs>
 <!---END-->
 
-**Add existing column to `DataFrame`:**
+### Create columns using intermediate result
 
-<!---FUN addExisting-->
+<!---FUN addCalculated-->
 
 ```kotlin
-val score by columnOf(4, 3, 5, 2, 1, 3, 5)
-
-df.add(score)
-df + score
+val personWithCityInfo = df.add {
+    val cityInfo = city.map { queryCityInfo(it) }
+    "cityInfo" {
+        cityInfo.map { it.location } into CityInfo::location
+        cityInfo.map { it.population } into "population"
+    }
+}
 ```
 
 <!---END-->
 
-**Add all columns from another `DataFrame`:**
+## Add existing column to `DataFrame`
 
-<!---FUN addDfs-->
+<!---FUN addExisting-->
 
 ```kotlin
-df.add(df1, df2)
+val score by columnOf(4, 3, 5, 2, 1, 3, 5)
+
+df.add(score)
+df + score
 ```
 
 <!---END-->
 
-**Add columns using intermediate result:**
+## Add all columns from another `DataFrame`
 
-<!---FUN addCalculated-->
+<!---FUN addDfs-->
 
 ```kotlin
-val personWithCityInfo = df.add {
-    val cityInfo = city.map { queryCityInfo(it) }
-    "cityInfo" {
-        cityInfo.map { it.location } into CityInfo::location
-        cityInfo.map { it.population } into "population"
-    }
-}
+df.add(df1, df2)
 ```
 
 <!---END-->