docs: document time_of_last_update in /ping response #257

Job	Run time
Preflight Check	4s
Resolve Baseline Tag	6s
Compat tests skipped (fork PR)	0s
Compat (services) — new code vs v1.9.1 tests	28s
Compat (evaluation) — new code vs v1.9.1 tests	1m 37s
Compat (memory) — new code vs v1.9.1 tests	11m 57s
Compat (runtime) — new code vs v1.9.1 tests	30s
	14m 42s

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