Fix docstring format errors found by ruff (Qiskit#9708)

* Remove blank lines before class docstring, ruff D211

* Capitalize first word of docstrings, ruff D403

* Remove empty docstrings, ruff 419

* Remove blank line before function docstring, ruff D201

* Add colon to docstring section names, ruff D416

* Move closing triple quote in docstring to newline, ruff D208

The ruff rule D208 is actually "Docstring is over-indented". In fact they are
not overindented. But the error is somehow triggered because there is
no newline fore the closing triple quote. Adding this newline silences the
error.

* Use canonical capitalizatio of section names in docstrings, ruff D405

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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/algorithms/time_evolvers/pvqd/pvqd.py

@@ -216,7 +216,6 @@ def get_loss(
         dt: float,
         current_parameters: np.ndarray,
     ) -> tuple[Callable[[np.ndarray], float], Callable[[np.ndarray], np.ndarray]] | None:
-
         """Get a function to evaluate the infidelity between Trotter step and ansatz.
 
         Args: