Fix docs URL

`rtd.io` is on longer operational, according to [the project page](https://readthedocs.org/projects/mss/) the correct domain is either `rtfd.io` or the full `readthedocs.io`. Also switch to https.

README.md

@@ -74,7 +74,7 @@ Current release info
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.6572620.svg)](https://doi.org/10.5281/zenodo.6572620)
 [![JuRSE Code Pick](https://img.shields.io/badge/JuRSE_Code_Pick-July_2024-blue)](https://www.fz-juelich.de/en/rse/jurse-community/jurse-code-of-the-month/july-2024)
 [![Conda Platforms](https://img.shields.io/conda/pn/conda-forge/mss.svg)](https://anaconda.org/conda-forge/mss)
-[![DOCS](https://img.shields.io/badge/%F0%9F%95%AE-docs-green.svg)](http://mss.rtd.io)
+[![DOCS](https://img.shields.io/badge/%F0%9F%95%AE-docs-green.svg)](https://mss.rtfd.io)
 [![Conda Recipe](https://img.shields.io/badge/recipe-mss-green.svg)](https://anaconda.org/conda-forge/mss)
 [![Conda Downloads](https://img.shields.io/conda/dn/conda-forge/mss.svg)](https://anaconda.org/conda-forge/mss)
 [![Coverage Status](https://coveralls.io/repos/github/Open-MSS/MSS/badge.svg?branch=develop)](https://coveralls.io/github/Open-MSS/MSS?branch=develop)