Cleanup website with following (#276)

1. Removed releases documentation as those are very old. New release documentation can be found at https://github.com/babelfish-for-postgresql/babelfish-for-postgresql/releases
2. Removed section on "Unsupported functionality"
3. Removed https://babelfishpg.org/docs/usage/limitations-of-babelfish

Signed-off-by: Shard Gupta <shardga@amazon.com>

Author: shardgupta

DOC_STRUCTURE.md

@@ -25,10 +25,7 @@ just_the_docs:
     faq:
       name: FAQ and getting help
       nav_fold: true
-    versions:
-      name: Releases
-      nav_fold: true
-      order: desc
+
 ```
 
 Each one of the `collections` attributes points to the folder in which the content resides, prefixed with an underscore.  For example: