gh-134789: Document del s[i] operation for mutable sequences

[rishabh11336]

Issue #134789

• 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 “del s[i]” not mentioned under “Mutable Sequence Types” #134789.

• Issue: “del s[i]” not mentioned under “Mutable Sequence Types” #134789

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