Improve docs build ergonomics

[jl-wynen]

This PR makes a couple of changes:

• It adds missing paths to gitignore
• It moves the doctrees into the packages so that they don't collide with each other in /tmp (Plus, /tmp is a Unix path)
• It adds a separate docs task for each package. Now we can run the docs using

pixi run docs-essimaging

Unfortunately, I could not find a way to reuse the sphinx command and also automatically activate the right environment.

How this works

Take for example ESSreduce: We have a docs-essreduce feature. This feature contains a task also called docs-essreduce. This connects the task to that feature. Since we only have one environment (also called ess-reduce) that contains the docs-essreduce feature, pixi knows to use that environment for this task. (And all tasks in this feature.)
This can be confusing, especially since there are 3 different things with the same name :/

[jl-wynen]

Correction: I had to add default-environment to the docs-essreduce task because the docs-essreduce feature appears in all docs-* environments and pixi was unable to figure out which env to use on its own.