Improve accessibility of Parcels to new contributors

[VeckoTheGecko]

Summary of changes needed

• Welcome bot
• Pixi workflows

Welcome bot

Similar to what Xarray has, add a bot to welcome new contributors to Parcels.

https://github.com/apps/welcome

https://github.com/pydata/xarray/blob/9bee765f8846194778c7069bc913c07c83e7443c/.github/config.yml

I think we should have the following:

• Welcome message for new issue submitters
• Welcome message for new PR submitters (i.e., don't worry about the check marks - we'll help you along)

I think that should be sufficient?

Pixi workflows

Update the CI workflows to rely more on the Pixi tasks. Then add a pixi run ci to "run the CI".