[documentation] add guide/readme on how to integrate additional space/parcellations/maps/features

[xgui3783]

One of the user feedback is, how to include their own data on:

• [ ] their own machine
• [ ] push into siibra ecosystem, so everyone using siibra could use them

We would ideally like have a readme/rst, which should contain:

• [ ] how to point to local directory (env var)
• [ ] where to add the json file (e.g. parcellation go ./parcellations/ etc)
• [ ] how to validate added json
• [ ] how to verify that added json is used by siibra

[AhmetNSimsek]

Here is a the first draft for creating/using preconfigs: https://github.com/FZJ-INM1-BDA/siibra-python/blob/main/docs/create_preconfiguration.rst