docs: describe how to generate secret from cert file

[ps-19]

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:

This PR addresses issue #1767, which highlights a lack of documentation on how to generate a Kubernetes secret from a cert file. Specifically, the current documentation for protecting a route using SSL with the Apache APISIX Ingress Controller does not provide information on the format and meaning of the secret or how to generate it.

To address this issue, this PR adds detailed instructions on generating a Kubernetes secret from a cert file. The updated documentation includes the meaning and format of the secret, as well as commands to generate the base64 value of the cert and private key files. With these instructions, users will be able to properly generate a secret and use it to protect their routes using SSL with the Apache APISIX Ingress Controller.

Overall, this PR improves the usability and accessibility of the Apache APISIX Ingress Controller documentation by providing clear instructions on an important aspect of SSL configuration.

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

[kingluo]

The changes are ok with me. Please ensure passing the CI.