Drizzlepac/HAP: Add documentation for astrometry-related env variables

[stscijgbot-hstdp]

Issue HLA-1473 was created on JIRA by Steve Goldman:

There was some confusion regarding a couple environmental variables associated with HAP products. The variables are:

• ASTROMETRIC_CATALOG_URL: ** used by drizzlepac (link)

•  ASTROMETRY_SERVICE_URL: ** used by STWCS (link)

More details on these environmental variables should be included in the Drizzlepac/HAP readthedocs documentation.