Remove “Documentation” page (#466)

The only purpose of the “Documentation” page is to link to the Flume User Guide and Developer Guide. Those links are already in the navigation bar, so the page is redundant.

Author: ppkarwasz

src/site/antora/modules/ROOT/nav.adoc

@@ -19,7 +19,7 @@
 * xref:getinvolved.adoc[Get Involved]
 * xref:download.adoc[Download]
 * xref:security.adoc[Security]
-* xref:documentation.adoc[Documentation]
+* Documentation
 ** xref:FlumeUserGuide.adoc[Flume User Guide]
 ** xref:FlumeDeveloperGuide.adoc[Flume Developer Guide]
 * xref:releases/index.adoc[Releases]