Make unitless quantity recommendation more explicit

Currently, our docs recommend using "the empty string" for unitless quantities (see https://data-central.readthedocs.io/en/latest/data_types/units.html#encoding-dimensionless-or-unitless-quantities). For me as a reader, it wasn't 100% clear whether that meant using ""/'', or leaving a blank in the psv (i.e. || pipes surrounding the empty unit field). Additionally, we have an example table (https://data-central.readthedocs.io/en/latest/data_types/catalogues.html#survey-datarelease-column-meta-txt) that leaves it blank, and does not use the empty string. We need to unify these, and make it more clear. The description should come with an example, and the unit attribute description should include the description too.

Liz has a preference for empty quotations, not an empty field (i.e., "" or '' is correct) as postgres assumes a "null" string is an empty string. This task might need some investigation/discussion with the ingestion team to figure out exactly what is correct/preferred.