update generate bundle script to support flat index files

[tomkralidis]

The generate bundle script needs to be updated to support the "flat" index approach implemented in:

• https://github.com/wmo-im/wis2-topic-hierarchy/tree/main/topic-hierarchy/earth-system-discipline/weather/space-based-observations
• https://github.com/wmo-im/wis2-topic-hierarchy/tree/main/topic-hierarchy/earth-system-discipline/space-weather/space-based-observations

Note that the bundle is made available at the following locations:

• https://schemas.wmo.int/wth/a/bundle
• https://wmo-im.github.io/wis2-topic-hierarchy