Add STORAGE_<LOCATION>_HEALTHCHECK_WRITE_INTERVAL to docs

[NickNovoselov]

Add STORAGE_<LOCATION>_HEALTHCHECK_WRITE_INTERVAL environment variable documentation

Summary

Documents the new optional environment variable STORAGE_<LOCATION>_HEALTHCHECK_WRITE_INTERVAL. This variable allows users to throttle health‑check write operations (PUT) to cloud storage backends (like S3), dramatically reducing the number of PUT requests and helping stay within free‑tier limits.

Related PR

Link to the corresponding PR in the main Directus repository (once created).

Changes

• Added a row to the Storage configuration describing the new variable, its behaviour, and its default value (0 – meaning always write).
• Clarified that the health check reads the existing health‑check file (GET) during the interval, and only performs a full write (PUT) when the interval expires or the read fails.
• Provided guidance that setting the variable to a non‑zero value (e.g. 60000 for one minute) significantly cuts PUT operations while still reliably monitoring storage health.

Checklist

• Variable name matches the implementation
• Description accurately reflects the throttling and fallback logic
• Default value documented
• Awaiting merge of the main Directus PR (or PR number for cross‑reference)

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