Add crucial information to the SparseHamiltonian doc string

[Qottmann]

Context:

Added reference to qml.utils.sparse_hamiltonian to qml.SparseHamiltonian class and a use-case example.

Description of the Change:

Just changed the doc-string

Benefits:

Now it should be clear to the user how to construct sparse Hamiltonians using PennyLane.

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2572 (25677b8) into master (c1db150) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2572   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         249      249           
  Lines       20135    20135           
=======================================
  Hits        20052    20052           
  Misses         83       83           

Impacted Files	Coverage Δ
pennylane/ops/qubit/observables.py	100.00% <ø> (ø)

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[josh146]

Thanks @Qottmann! Looks 💯, really nice example

[antalszava]

Hi @Qottmann, just double-checking: would this be good to be merged? 🙂