Update SGW 3.0. docs to 3.0.x

From CBG-2116

Note that in addition to removing the builds and updating the downloads pages,  there are docs related changes that will need to be coordinated. The first two in list are important.  The #3 can come in after we remove as well. 

1. The release notes for 3.0 should indicate that this build has been replaced with 3.0.3 which includes critical bug fixes . We shoulnt remove release notes https://docs.couchbase.com/sync-gateway/current/release-notes.html#lbl-rel300
2. https://docs.couchbase.com/sync-gateway/current/compatibility.html - should specify 3.0.3 and not 3.0.0
3. More generally- scan docs and Replace all occurrences of 3.0.0 to 3.0.3. Or just go with 3.0.x to keep future proof