docs: fix broken links and add CI for checking document build status

[ChenYi015]

What this PR does / why we need it:

• Fix broken links in tensorflow/triton serving documentation
• Add CI for checking whether docs can be built correctly without errors and warnings

Rational:

There is some broken doc links when building arena documentation:

$ mkdocs build --strict
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /Users/chenyi/code/github.com/kubeflow/arena/site
...
WARNING -  Doc file 'serving/tfserving/update-serving.md' contains a link 'tfserving/gpu.md', but the target 'serving/tfserving/tfserving/gpu.md' is not found among documentation files.
WARNING -  Doc file 'serving/triton/update-serving.md' contains a link 'triton/serving.md', but the target 'serving/triton/triton/serving.md' is not found among documentation files.

Aborted with 2 warnings in strict mode!

[ChenYi015]

/assign @Syulin7 @cheyang

[cheyang]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheyang

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