docs: Move BuildElement's "Built-in functionality" to a new page

The documentation of how to use the common features provided by the
`BuildElement` base class is currently part of the API reference even
though it's more relevant to project authors than plugin developers.

This moves that section to a new page in the "Reference" section, next
to other plugin-specific documentation.

Co-authored-by: Dom Rodriguez <dom.rodriguez@codethink.co.uk>