[main] Update stdtypes.rst
- 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.
| ``s[i] = x`` | item *i* of *s* is replaced by | |
| | *x* | |
+------------------------------+--------------------------------+---------------------+
+| ``del s[i]`` | removes item *i* of *s* | |
++------------------------------+--------------------------------+---------------------+
| ``s[i:j] = t`` | slice of *s* from *i* to *j* | |
| | is replaced by the contents of | |
| | the iterable *t* | |