[3.14] gh-134789: Document del s[i] operation for mutable sequences (GH-134804) ...

[3.14] gh-134789: Document del s[i] operation for mutable sequences (GH-134804) (#134808)

gh-134789: Document del s[i] operation for mutable sequences (GH-134804)

[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 GH-134789.
(cherry picked from commit 967f361993c9c97eb3ff3076a409b78ea32938df)

Co-authored-by: Rishabh Singh <67859818+rishabh11336@users.noreply.github.com>