Mapping: Consistently document and present the mappings for 'core' and sustainability CoST IDS elements

[duncandewhurst]

The mapping for 'core' CoST IDS elements is presently documented in the CSV files in the mapping directory. For consistency and ease of editing and validation (see #502), we should switch to using the YAML format in which the mapping for the sustainability modules is documented.

For ease of comprehension, we should present the mappings for the 'core' elements in the documentation site in the same format as the mappings for the sustainability elements.