Update documentation to match changes to BP file finding

Kenneth Morelandrequested to merge
kmorel/fides:docs-data-source-no-path into master

Recent changes support specifying a BP file from a JSON file relative to the location of the JSON file. Although the old method of specifying paths is still supported, this new method is preferrable and should be documented.

The documentation for the schema and the C++ reader interface are updated to document this new method.

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