Add cross-platform conda env manifest and describe use

Author: sdruskat

README.md

@@ -4,7 +4,10 @@ This repository contains the source files for the de-RSE website. Its official h
 
 The site is made to be built with [Jekyll](https://jekyllrb.com/) >= 3.4.1.
 
-You can use [RVM](https://rvm.io/) to install and use a suitable Ruby version, e.g., `rvm use 2.7`.
+You can use, e.g.,
+
+- [RVM](https://rvm.io/) to install and use a suitable Ruby version, e.g., `rvm use 2.7`.
+- [`conda`/`mamba`] to install dependencies into a reproducible environment via `mamba env create -f conda-env-de-rse-website.yml && mamba activate de-rse-website`.
 
 To build, run `bundle install` once (or skip that if you have dependencies already installed and bundler does not work for you) and then  `bundle exec jekyll build`.
 To preview locally, run `bundle exec jekyll serve --incremental` and browse to <http://localhost:4000>.

conda-env-de-rse-website.yml

@@ -0,0 +1,8 @@
+name: de-rse-website
+channels:
+  - conda-forge
+dependencies:
+  - c-compiler
+  - compilers
+  - cxx-compiler
+  - ruby