docs: fix contributing links (#1647)

Author: janfb

CONTRIBUTING.md

@@ -1,5 +1,5 @@
 # Contributing to sbi
 
-The latest contributing guide is available in the repository at [`docs/contribute.md`](docs/contribute.md), or online at:
+The latest contributing guide is available in the repository at [`docs/contributing.md`](docs/contributing.md), or online at:
 
-[sbi-dev.github.io/sbi/latest/contribute/](https://sbi-dev.github.io/sbi/latest/contribute/)
+[sbi.readthedocs.io/en/latest/contributing.html](https://sbi.readthedocs.io/en/latest/contributing.html)

README.md

@@ -1,6 +1,6 @@
 [![PyPI version](https://badge.fury.io/py/sbi.svg)](https://badge.fury.io/py/sbi)
 [![Conda Version](https://img.shields.io/conda/vn/conda-forge/sbi.svg)](https://github.com/conda-forge/sbi-feedstock)
-[![Contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](https://github.com/sbi-dev/sbi/blob/master/CONTRIBUTING.md)
+[![Contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](https://sbi.readthedocs.io/en/latest/contributing.html)
 [![Tests](https://github.com/sbi-dev/sbi/actions/workflows/ci.yml/badge.svg)](https://github.com/sbi-dev/sbi/actions)
 [![codecov](https://codecov.io/gh/sbi-dev/sbi/branch/main/graph/badge.svg)](https://codecov.io/gh/sbi-dev/sbi)
 [![GitHub license](https://img.shields.io/github/license/sbi-dev/sbi)](https://github.com/sbi-dev/sbi/blob/master/LICENSE.txt)