docs: order past events in descending order

[jtcrowson]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Follow up to #1617

• Upcoming events are in soonest to latest order (JSON ordering, not enforced)
• Past events are in least recent to most recent order (JSON ordering, not enforced)

What is the new behavior?

Enforces ordering on the events page:

• Upcoming events are in soonest to latest order (enforced)
• Past events are in most recent to least recent order (enforced)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for d30bc4e at https://previews.ngrx.io/pr1695-d30bc4e/

[brandonroberts]

What about just 'ascending' and `descending'?

[jtcrowson]

Fixed in 26275a5.

[ngrxbot]

Preview docs changes for 26275a5 at https://previews.ngrx.io/pr1695-26275a5/

[ngrxbot]

Preview docs changes for 6b9160b at https://previews.ngrx.io/pr1695-6b9160b/