Add explanation of SparsePauliOp

[beckykd]

URL to the relevant documentation

The operator page: https://docs.quantum.ibm.com/build/operators-overview does not cover SparsePauliOp that is the main operator class used in practice (comment from Paul N)

Select all that apply

• typo
• code bug
• out-of-date content
• broken link
• other

Describe the fix.

Add an explanation of SparsePauliOp