docs: split Lance Catalog from Lance Lakehouse

One page was carrying two layers. The catalog answers what exists and what it is
called -- names, hierarchy, resolution, and the grants that follow the
hierarchy. The lakehouse is everything that acts on those names: storing rows,
searching them, deriving new ones, and keeping it all fast.

Separating them makes the dependency visible. The catalog is a specification
another engine can implement, which is why a Lance table is reachable without
LanceDB; the lakehouse is where the cost and the tuning live. Governance belongs
to the catalog because the hierarchy is what a grant attaches to, and
maintenance belongs to the lakehouse because it is the files being rewritten.

Also fixes a corruption from an earlier bulk link rewrite: "central
metadata/catalog services" in prose had become a path.

docs/web/docs.json

@@ -167,6 +167,7 @@
             "pages": [
               "basics/lance",
               "basics/catalog",
+              "basics/lakehouse",
               "basics/query",
               "basics/jobs",
               "tables/consistency",