chore(number_theory/pell*): rename files, update doc (#18503)
Following a suggestion by @ocfnash in a comment to [#18484](https://github.com/leanprover-community/mathlib/pull/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).
