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the point that its own Next steps cards pointed at the pages that superseded it.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/sql/join.mdx

@@ -32,7 +32,7 @@ avoids repeating it in the select list.
 
 ## Across namespaces {#namespaces}
 
-Tables in different [namespaces](/namespaces) join by their dotted paths -- the
+Tables in different [namespaces](/management/namespace) join by their dotted paths -- the
 catalog is one catalog, so there is nothing to federate:
 
 ```sql