docs: add the tip for nydus-zran

[Desiki-high]

we should tell user the nydus zran image must have the same namespace with the oci image.

Fixs: #1223

[anolis-bot]

@Desiki-high , a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/68452

[anolis-bot]

@Desiki-high , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		❌ FAIL

Sorry, your test job failed. Please get the details in the link.

[anolis-bot]

@Desiki-high , the code has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/68465

[anolis-bot]

@Desiki-high , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!