Improve navigability for docs on infinispan.org

Update version drop-down menu for docs. Ex: from the 9.4.x User Guide, select 10.1.x from the version drop-down, get a 404. It should take you to the index page instead.

Also from older documentation versions page, add link to index for 10.x+ alongside javadocs, schema.

404 for the contributing guide link https://infinispan.org/getinvolved/ link should not point to https://infinispan.org/docs/stable/contributing/contributing.html