Add pandoc to the environment dependencies

[akorgor]

Building the user documentation with the provided conda environment recipes fails due to a missing pandoc dependency on a clean setup. This PR adds pandoc to the conda environment file.

[jessica-mitchell]

Hi @akorgor pandoc is a bit of a tricky situation because you can't use just any version -- you need the system install with apt package - the instructions for building the docs mention this too

[akorgor]

Hi @akorgor pandoc is a bit of a tricky situation because you can't use just any version -- you need the system install with apt package - the instructions for building the docs mention this too

Thank you @jessica-mitchell. Sorry, I was not aware of this part of the installation documentation.

[akorgor]

It might be possible to restrict the Pandoc version to the one available via apt. Conda currently provides 3.9 while apt provides 3.1.3, so this would simply require downgrading. The build with 3.9 completed without errors, but it is unclear whether parts of the documentation would need to be verified for correctness.

[jessica-mitchell]

Ok , it looks like installing pandoc with the version in conda-forge, works fine and everything builds as expected