chore(number_theory/pell*): rename files, update doc (#18503)

Following a suggestion by @ocfnash in a comment to [#18484](#18484), this
* renames `number_theory.pell` to `number_theory.pell_matiyasevic`
* renames `number_theory.pell_general` to `number_theory.pell`
* updates documentation accordingly
* moves the "TODO" note from `pell_matiyasevic` to `pell` (and updates it)

See also [this Zulip thread](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there-code-for-X.3F/topic/Proving.20Pell's.20equation.20is.20solvable/near/322885832).

src/number_theory/dioph.lean

@@ -6,7 +6,7 @@ Authors: Mario Carneiro
 import data.fin.fin2
 import data.pfun
 import data.vector3
-import number_theory.pell
+import number_theory.pell_matiyasevic
 
 /-!
 # Diophantine functions and Matiyasevic's theorem