DOC: update PyDarshan docs/examples to use context manager for report objects

[shanedsnyder]

See #851 for original issue and #858 for solution.

Long story short, it seems that using a context manager (i.e., with darshan.DarshanReport(log_path) as report:) for report objects helps avoid potential memory/threading issues when releasing them. This ultimately was causing some of our pytest runs to hang in various contexts.

We should update our docs and various examples to use this approach for managing report objects so users don't get potentially bitten by this.