chore: split Topology.Connected.Basic (#15144)

This splits `Topology.Connected.Basic` approximately in half, into `Basic` and `Clopen`. This removes the `Mathlib.Topology.Clopen` import from `Basic`, and allows for one more file import from #14927 in the `Clopen` file.

As far as declaration order goes, only the first 5 results in `Clopen` were moved "down" into the `Clopen` file, and only the last 4 results in `Basic` were moved "up" into the `Basic` file. Otherwise no changes except for new module documentation were made.

Author: j-loreaux

Mathlib.lean

@@ -4347,6 +4347,7 @@ import Mathlib.Topology.Compactness.PseudometrizableLindelof
 import Mathlib.Topology.Compactness.SigmaCompact
 import Mathlib.Topology.CompletelyRegular
 import Mathlib.Topology.Connected.Basic
+import Mathlib.Topology.Connected.Clopen
 import Mathlib.Topology.Connected.LocallyConnected
 import Mathlib.Topology.Connected.PathConnected
 import Mathlib.Topology.Connected.Separation