[FEA] Automatically Calculate Development days

[ajschmidt8]

https://docs.rapids.ai/maintainers

We should add a custom Jekyll filter/plugin that automatically calculates the duration shown on the page above.

We could potentially use this library: https://github.com/gocardless/business

This will make one less thing that we have to manually enter into releases.json.

[ajschmidt8]

In hindsight, we could probably just drop the development days count entirely.