docs: minor changes

[oliver-sanders]

Response to feedback from the recent training event. Highlights:

• Make "diff" code examples foolproof by removing (+| ) characters and (-) lines from copyable text.
• Visual simplification datetime recurrence slides.

Note that files in sphinx/_static/js will be run through eslint by the test battery.

[sadielbartholomew]

All good. The text-copying exclusion feature is a great idea that works as outlined.