Update and organize drought related recipes

[lukruh]

I found some unmaintained recipes related to droughts floating around in the recipe folder:

• recipe_consecdrydays.yml
• recipe_martin18grl.yml
• recipe_spei.yml

Some new ones are currently developed. We probably end up with 10 recipes or more. For me it would make sense to group them together in one "Droughts" category in the documentation. This hopefully makes it more organized and visible for users.
Would it also make sense to group the recipe files in a folder (recipes/droughts/...) similar to the diagnostics?

I already discussed with @katjaweigel, that we could start a first PR for updating the documentation and diagnostics to calculate PET, SPI and SPEI (which are used by most of the recipes). I am going to open a draft PR soon. I'd also like to bump the SPEI.R package to v1.8 in the requirements.

[lukruh]

I started over with some updated recipes and diagnostics in diag_scripts/droughts and recipes/droughts. When everything is there we can archive/deprecate the original recipes somehow.

I opened a draft PR to share the current code and documentation.