tiproxy: add note about reloading certificates once an hour (#22138)

[ti-chi-bot]

This is an automated cherry-pick of #22138

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References:

• https://github.com/pingcap/tiproxy/blob/d9ef65aa16552e7b76ed51d6bf5ce7c0a1504ecd/pkg/manager/cert/manager.go#L96-L121
• https://github.com/pingcap/tiproxy/blob/d9ef65aa16552e7b76ed51d6bf5ce7c0a1504ecd/pkg/manager/cert/manager.go#L21

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@dveeden This PR has conflicts, I have hold it. Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

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[gemini-code-assist[bot]]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the TiDB documentation to provide crucial information about how TiProxy manages TLS certificates. It specifically highlights that TiProxy reloads certificates from disk on an hourly basis, which means any modifications to certificate files will have a delay of up to one hour before they become active. This clarification aims to improve user understanding of certificate management within TiProxy.

Highlights

• Documentation Update: Added important information to the TiDB documentation regarding TiProxy's certificate reloading mechanism.
• Certificate Reload Frequency: Clarified that TiProxy reloads certificates from disk once every hour.
• Impact of Certificate Changes: Noted that any changes made to certificate files on disk might take up to one hour to take effect due to the hourly reload cycle.

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@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	4b99b89e3780968ce9b26360a716dc38bcf6e8ac	link	true	/test pull-verify

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