Add vignette on filtering on time periods

## Describe the solution you'd like

We need a good vignette to explain the subtleties / possibilities around selecting time periods using eesyapi:
- How to select a simple combination of possible time periods
- How to select all time periods before a given time period
- How to select all time periods after a given time period
- How to select all time periods between two given time periods

I'm thinking this should be linked appropriately from the workflow vignette.

## Additional context

Some of this isn't implemented yet, the work is outlined in #44

Linked equivalents for geographies (#43 ) and filter items (#45).