docs(discovery): add document for k8s mtls

[e1ijah1]

Description

K8s discovery with mTLS document for #8699

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[zhixiongdu027]

使用mTLS 一般是因为需要在 kubernetes 集群外部启动apisix

这种情况下，文档重点应该是描述

1. 从 kubernetes conf 文件中提取证书数据
2. 如何将证书数据填充到启动配置

[github-actions]

This pull request has been marked as stale due to 60 days of inactivity. It will be closed in 4 weeks if no further activity occurs. If you think that's incorrect or this pull request should instead be reviewed, please simply write any comment. Even if closed, you can still revive the PR at any time or discuss it on the [email protected] list. Thank you for your contributions.

[github-actions]

This pull request has been marked as stale due to 60 days of inactivity. It will be closed in 4 weeks if no further activity occurs. If you think that's incorrect or this pull request should instead be reviewed, please simply write any comment. Even if closed, you can still revive the PR at any time or discuss it on the [email protected] list. Thank you for your contributions.

[monkeyDluffy6017]

Close this pr due to long period of inactivity, feel free to reopen it