Added a link to the missed datset titanic.csv (#389)

* Add the dataset used in the documentation

* Added the links to the dataset

Author: zaleslaw

data/titanic.csv

@@ -48,3 +48,5 @@ val passengers = DataFrame.read("titanic.csv")
 ```
 
 <!---END-->
+
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).

docs/StardustDocs/topics/KPropertiesApi.md

@@ -128,6 +128,8 @@ df.add("lastName") { name.split(",").last() }
 
 </tabs>
 
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).
+
 # Comparing the APIs
 
 The [String API](stringApi.md) is the simplest and unsafest of them all. The main advantage of it is that it can be

docs/StardustDocs/topics/apiLevels.md

@@ -29,6 +29,8 @@ DataFrame.read("titanic.csv")
 
 <!---END-->
 
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).
+
 <warning>
 Note that it still doesn’t solve the problem of whether the column actually exists in a data frame, but reduces type casting.
 </warning>

docs/StardustDocs/topics/columnAccessorsApi.md

@@ -26,6 +26,8 @@ df.add("lastName") { name.split(",").last() }
 
 <!---END-->
 
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).
+
 In notebooks, extension properties are generated for [`DataSchema`](schemas.md) that is extracted from [`DataFrame`](DataFrame.md) 
 instance after REPL line execution. 
 After that [`DataFrame`](DataFrame.md)  variable is typed with its own [`DataSchema`](schemas.md), so only valid extension properties corresponding to actual columns in DataFrame will be allowed by the compiler and suggested by completion.

docs/StardustDocs/topics/extensionPropertiesApi.md

@@ -64,6 +64,8 @@ df.groupBy { pclass }.aggregate {
 }
 ```
 
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).
+
 **Create:**
 ```kotlin
 // create columns

docs/StardustDocs/topics/overview.md

@@ -19,6 +19,8 @@ DataFrame.read("titanic.csv")
 
 <!---END-->
 
+The `titanic.csv` file could be found [here](https://github.com/Kotlin/dataframe/blob/master/data/titanic.csv).
+
 <warning>
 Note that if data frame doesn’t contain column with the string provided, or you try to cast to the wrong type it will lead to runtime exception.
 </warning>