docs: improve toolchain/k3d docs #24308

Job	Run time
Build, test & lint UI code	2m 32s
Ensure Go modules synchronicity	17s
Build & cache Go code	5m 5s
Check changes to generated code	14m 22s
Lint Go code	1m 9s
Run unit tests for Go packages	7m 33s
Run end-to-end tests (v1.28.2)	1h 4m 22s
Run unit tests with -race for Go packages	15m 48s
Run end-to-end tests (v1.27.6)	58m 35s
Run end-to-end tests (v1.26.9)	54m 55s
Run end-to-end tests (v1.25.14)	53m 55s
Process & analyze test artifacts	26s
	4h 38m 59s

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