refactor: move the public host out to vantage-site and vantage-docs

site/ and sitesvc/ become web/ and server/ in vantage-site; docsite/
becomes the root of vantage-docs. Their images move with them, to
vantage/vantage-site/{web,server} and vantage/vantage-docs.

Nothing here imported any of them, and sitesvc turned out to read no
database at all, so both cuts are clean. docker-compose.site.yml is
deleted rather than emptied: every service it held now ships with the
repository that builds it, and deploy/docker/docker-compose.yml is once
again exactly a self-hosted install.

Corrects four comments that named sitesvc for work it no longer does.

server/internal/models/instance.go

@@ -2,6 +2,6 @@ package models
 
 import shared "gitea.hostxtra.co.uk/vantage/vantage-shared/models"
 
-// Instance is defined in the shared module because sitesvc and the admin
-// control plane write the same documents.
+// Instance is defined in the shared module because the control plane and
+// Vantage HQ write the same documents from separate repositories.
 type Instance = shared.Instance