Update "Data Retention" to "Storage Proving"

https://github.com/FilOzone/dealbot/blob/main/docs/checks/data-retention.md#why-is-this-called-data-retention-vs-data-availability

Right now, the top-level metric around "retention" implies data loss / deletion in the context data is not "retained" -- however the metric is actually measuring proving reliability, where gaps could indicate node uptime or proving ability instead of data retention. Instead of using the word "retention", we should use a word that more clearly maps to "proving" since that is the onchain process that is being measured and sets a clearer auditor expectation on network and SP health.

Start with docs/checks/data-retention.md, especially the “Why is this called data retention vs data availability?” section linked in the issue. Review how the top-level metric is described, update the terminology to “Storage Proving” and explain the proving-reliability focus. Done means the documentation no longer implies that the metric directly measures data loss or deletion.

Written by the indexing model from the issue text.