Failing conda env creation command in installation docs

The docs state the following for installing httomo into a conda environment: https://github.com/DiamondLightSource/httomo/blob/dd8787459c4c4b7a5c566567803eacf4bfb70939/docs/source/howto/installation.rst?plain=1#L37-L41

However, the very first line fails:
```
conda env create --name httomo
```

because by default it is expecting to be given the path to an environment YAML file.

These docs should be updated in some manner to reflect a working way to create the conda env necessary for installation.