docs: one page per object in Manage

Namespace and Table were the only objects behind a nested group. Both are now a
single page like Database, Materialized view, View and Function, so Manage is a
flat list of the things a catalog holds.

Namespace merges its two pages, which were a concept page and a usage page for
the same object.

Table is a new page. The old tables/index was a 672-line basic-usage tutorial
whose every section had acquired a better home during the restructure --
connecting, ingesting, searching, filtering, adding and dropping columns -- to
the point that its own Next steps cards pointed at the pages that superseded it.
It is replaced by a table object page: what a table carries, create, open, list,
rename, clone, drop, and links out.

Schema evolution, versioning and branching stay as siblings rather than folding
in. Merging them would have made one 1,800-line page, which is not what the
other object pages look like.

docs/web/docs.json

@@ -90,6 +90,18 @@
     {
       "source": "/tutorials/feature-engineering",
       "destination": "/tutorials/feature-engineering/feature-engineering-101"
+    },
+    {
+      "source": "/tables",
+      "destination": "/management/table"
+    },
+    {
+      "source": "/namespaces",
+      "destination": "/management/namespace"
+    },
+    {
+      "source": "/namespaces/usage",
+      "destination": "/management/namespace"
     }
   ],
   "navigation": {
@@ -124,22 +136,11 @@
             "pages": [
               "management/index",
               "management/database",
-              {
-                "group": "Namespace",
-                "pages": [
-                  "namespaces/index",
-                  "namespaces/usage"
-                ]
-              },
-              {
-                "group": "Table",
-                "pages": [
-                  "tables/index",
-                  "tables/schema",
-                  "tables/versioning",
-                  "tables/branching"
-                ]
-              },
+              "management/namespace",
+              "management/table",
+              "tables/schema",
+              "tables/versioning",
+              "tables/branching",
               "management/materialized-view",
               "management/function"
             ]