Remove incomplete documentation topics (#681)

Three incomplete documentation topics, namely "Datetime Columns", "Special column types", and "String Columns", have been removed from the StardustDocs. These deletions have been reflected in the documentation tree and the sitemap in order to keep everything updated and avoid confusion for users.

Author: zaleslaw

docs/StardustDocs/d.tree

@@ -173,10 +173,6 @@
         <toc-element topic="columnOperations.md">
             <toc-element topic="columnArithmetics.md" toc-title="Arithmetics"/>
             <toc-element topic="columnStatistics.md" toc-title="Statistics"/>
-            <toc-element topic="specialColumnTypes.md">
-                <toc-element topic="stringColumns.md" toc-title="String"/>
-                <toc-element topic="datetimeColumns.md" toc-title="DateTime"/>
-            </toc-element>
         </toc-element>
         <toc-element topic="ColumnSelectors.md"/>
         <toc-element topic="collectionsInterop.md">

docs/StardustDocs/sitemap.xml

@@ -527,11 +527,6 @@
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
   <priority>0.41</priority>
 </url>
-<url>
-  <loc>https://kotlin.github.io/dataframe/datetimecolumns.html</loc>
-  <lastmod>2023-06-26T12:33:14+00:00</lastmod>
-  <priority>0.41</priority>
-</url>
 <url>
   <loc>https://kotlin.github.io/dataframe/moverename.html</loc>
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
@@ -692,11 +687,6 @@
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
   <priority>0.41</priority>
 </url>
-<url>
-  <loc>https://kotlin.github.io/dataframe/stringcolumns.html</loc>
-  <lastmod>2023-06-26T12:33:14+00:00</lastmod>
-  <priority>0.33</priority>
-</url>
 <url>
   <loc>https://kotlin.github.io/dataframe/maxby.html</loc>
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
@@ -712,16 +702,9 @@
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
   <priority>0.33</priority>
 </url>
-<url>
-  <loc>https://kotlin.github.io/dataframe/specialcolumntypes.html</loc>
-  <lastmod>2023-06-26T12:33:14+00:00</lastmod>
-  <priority>0.26</priority>
-</url>
 <url>
   <loc>https://kotlin.github.io/dataframe/columnstatistics.html</loc>
   <lastmod>2023-06-26T12:33:14+00:00</lastmod>
   <priority>0.26</priority>
 </url>
-
-
 </urlset>