CASSANDRA-21342: Fix broken Antora xref/anchor targets in cassandra docs

Forward-merge of the Phase 3 A2 fix from trunk (part of commit
32826fe) to cassandra-4.0. Retargets the ROOT-nav and ROOT-index
Contact us xrefs from the removed master@_:ROOT:contactus.adoc page
to the existing master@_:ROOT:community.adoc.

The fix is applied as a direct edit rather than a cherry-pick: 4.0's
ROOT/nav.adoc has a different surrounding structure from trunk's (no
ifndef::local-build wrapper, no development sub-nav), so a clean
cherry-pick of the trunk patch is not possible. The net effect on
the targeted xrefs is identical.

 patch by Patrick McFadin; reviewed by TBD for CASSANDRA-21342

Author: pmcfadin

doc/modules/ROOT/nav.adoc

@@ -1,4 +1,4 @@
 * xref:index.adoc[Main]
 ** xref:master@_:ROOT:glossary.adoc[Glossary]
 ** xref:master@_:ROOT:bugs.adoc[How to report bugs]
-** xref:master@_:ROOT:contactus.adoc[Contact us]
+** xref:master@_:ROOT:community.adoc[Contact us]

doc/modules/ROOT/pages/index.adoc

@@ -43,7 +43,7 @@ to do so by submitting your contribution like any other patch following
 
 == Meta information
 * xref:master@_:ROOT:bugs.adoc[Reporting bugs]
-* xref:master@_:ROOT:contactus.adoc[Contact us]
+* xref:master@_:ROOT:community.adoc[Contact us]
 * xref:master@_:ROOT:development/index.adoc[Contributing code]
 * xref:master@_:ROOT:docdev/index.adoc[Contributing to the docs]
 * xref:master@_:ROOT:community.adoc[Community]