Add conda installation instructions

This MR updates the documentation with installation instructions using conda instead of pip.

Can this MR be accepted?

• conda-forge recipe available (see https://github.com/conda-forge/staged-recipes/pull/12077 )
• Added conda-forge badge to GitLab project, see shields.io
• README updated
  • ... with installation instructions
  • ... with minor tweaks, fixes, and additions here and there

• Ready for merging
  • Pipeline passes without warnings
  • Squash option set
  • Changelog entry added
  • Version number bumped
  • (Tested &) approved by @jeremiastraub

Related issues

Closes #184 (closed)