Update stdtypes.rst

Issue #134789

Document del s[i] operation for mutable sequences

- Added explicit mention of `del s[i]` (item deletion by index) to the Mutable Sequence Types section.
- Clarified that this operation removes the item at the specified index from the sequence.
- Addresses issue #134789.

Author: rishabh11336

Doc/library/stdtypes.rst

@@ -1218,6 +1218,8 @@ accepts integers that meet the value restriction ``0 <= x <= 255``).
 |                              | is replaced by the contents of |                     |
 |                              | the iterable *t*               |                     |
 +------------------------------+--------------------------------+---------------------+
+| ``del s[i]``                 | removes item i from s          |                     |
++------------------------------+--------------------------------+---------------------+
 | ``del s[i:j]``               | same as ``s[i:j] = []``        |                     |
 +------------------------------+--------------------------------+---------------------+
 | ``s[i:j:k] = t``             | the elements of ``s[i:j:k]``   | \(1)                |