5.x Client Catalog API and Docs

[lesserwhirls]

API docs need to be added to several of the classes down in thredds.catalog.client. For example, in:

https://github.com/Unidata/netcdf-java/blob/master/cdm/core/src/main/java/thredds/client/catalog/Catalog.java

The docs should at least point out that you need to use the builders:

https://github.com/Unidata/netcdf-java/tree/master/cdm/core/src/main/java/thredds/client/catalog/builder

(specifically CatalogBuilder for those wanting to read a THREDDS Client Catalog).

Also need to move https://github.com/Unidata/netcdf-java/blob/master/docs/src/private/website/netcdf-java/reference/ThreddsCatalogs.adoc into the main documentation set, as well as beef it up with some tangible examples (e.g. working with THREDDS Metadata, following catalogRef's).

Need to add thredds.catalog.client to the public API generation so that it is accessible via https://docs.unidata.ucar.edu/netcdf-java/<version>/javadoc/