Replace (almost) all matrix-org/synapse references with element-hq/synapse

Issues and Pull Requests were not migrated to the new
organization/repository, so `matrix-org/synapse/pull` and
`matrix-org/synapse/issues` references were kept as-is.

`matrix-org/synapse-s3-storage-provider` references were also kept,
as that module still continues living under the `matrix-org` organization.

This patch mainly aims to change documentation-related things, not actual
usage in full yet. For polish that, another more comprehensive patch is coming later.

roles/custom/matrix-bot-go-neb/defaults/main.yml

@@ -290,13 +290,13 @@ matrix_bot_go_neb_services: []
 #      Rooms:
 #        "!someroom:id":
 #          Repos:
-#            "matrix-org/synapse":
+#            "element-hq/synapse":
 #              Events: ["push", "issues"]
 #            "matrix-org/dendron":
 #              Events: ["pull_request"]
 #        "!anotherroom:id":
 #          Repos:
-#            "matrix-org/synapse":
+#            "element-hq/synapse":
 #              Events: ["push", "issues"]
 #            "matrix-org/dendron":
 #              Events: ["pull_request"]