Bugfix/website cmake specifications

**Short description** Upon trying to install GQCP myself, I noticed a few issues with the current website documentation for installing as a developer. **To do** - [x] Refactor `cmake` call to ensure no whitespaces are added after `\` (this breaks the bash newline) - [x] prompt user to check if `$CONDA_PREFIX` is not empty, as if this is the case the `cmake` command gives an error.