Convert config file to yaml

[deining]

The docsy user guide uses yaml format for its config files, while the example site still uses config.toml.
According to this issue, yaml seems to be the preferred format for the docsy project and its repos. Therefore we should:

• convert the config file of the example site to YAML format
  hugo.yaml from toml config, preserving comments #299
  and
• reflect these change in the docsy user guide
  --> moved to [UG] Ensure that Docsy Example config excerpts are consistent with new hugo.yaml file docsy#2008
• Update hugo.toml in prep for conversion #297
  • Update hugo.toml: revert taxonomyCloud elt order #298