Only attempt RTD build for upstream

[hugovk]

The build fails for forks because they don't have secrets, so skip that step for forks.

For example:

Error: Input required and not supplied: webhook_url

https://github.com/hugovk/siibra-python/actions/runs/6909815054/job/18801835122

[xgui3783]

LGTM

apologies, we are in the middle of administrative tasks, and may not be very responsive until sometime next week.

[hugovk]

Hello, reminder for this :)

[hugovk]

Closing this year-old PR, happy to re-open if there's any interest.