Update repo links in readthedocs

[TeddybearCrisis]

Description

When clicking links in the documentation I end up at source code in the old repo (https://github.com/wjakob/nanogui).
This is no big deal but a bit confusing for newcomers. If the docs are shared between repos a hint could help.

Experienced

1. Opened this documentation page
2. A click on the "Example 1 in C++" link gets me to the wjakob repo example

Expected

2. A click on the "Example 1 in C++" link should get me to the mitsuba-renderer repo example

This experience goes for all the example sources hyperlinks.

Thanks

Amazing project btw. Having spent some time in MMSPG it's nice to see EPFL creating cool stuff.

[butla]

This is a bigger deal for Python examples, which are using outdated class/field names and don't run.