Fix previousRelativeTo, document the public API, release 0.3.1

[justintout]

Brings dartdoc coverage from 17/19 to 19/19 public API elements (pana), fixes a previousRelativeTo bug found while documenting it, and bumps the version to 0.3.1.

Docs:

• Rewrites every doc comment to state what the member returns, instead of opening with the member's name.
• Documents what the Cron constructor accepts and when it throws, including @reboot and expressions that can never match.
• Adds a usage example to the class doc. The output shown was checked by running it.
• States that nextRelativeTo and previousRelativeTo never return the given time, and how long a rare schedule's search takes. The worst case found was about 0.9s, for a Feb 29 schedule across 2100.
• Adds pub.dev version and docs badges to the README.
• dart doc --dry-run reports 0 warnings.

Fix:

• previousRelativeTo started its search a full minute plus the leftover seconds before the given time, so it skipped a match earlier in the same minute. */15 * * * * at 16:00:30 returned 15:45 instead of 16:00. previous, sincePreviousRelativeTo and sincePrevious had the same bug. Four existing test rows asserted the old result and are corrected.

Release:

• pubspec.yaml 0.3.1 and its CHANGELOG entry. After merging, run Release on main.