Update docs for datapoint retention configuration #165

The documentation still refers to the old datapoint retention configuration:

• OR_DATA_POINTS_MAX_AGE_DAYS
• dataPointsMaxAgeDays

Since openremote/openremote#2352, datapoint retention is configured in weeks instead of days. The docs should be updated to use OR_DATA_POINTS_MAX_AGE_WEEKS and remove or clarify references to the old per-attribute dataPointsMaxAgeDays setting, which no longer applies.

Known outdated reference:

• Where's the data stored? section

Acceptance criteria:

• Replace outdated OR_DATA_POINTS_MAX_AGE_DAYS references with OR_DATA_POINTS_MAX_AGE_WEEKS
• Remove or update dataPointsMaxAgeDays documentation
• Ensure the docs clearly describe the current retention behaviour