docs: specify namespace in metadata explicitly

[kayx23]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Currently the doc explicitly sets the preferred namespace with kubectl config set-context --current --namespace=ingress-apisix and leaves out the namespace specification in manifest file. However if users did not read the docs sequentially and set the preferred namespace, resources could be applied to the wrong namespace, leading to downstream issues.

This PR explicitly sets namespace in metadata.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first