docs: update READMEs with new APIs and usage examples #938

Job	Run time
parser-tests (pgsql-cli)	54s
parser-tests (deparser)	1m 0s
parser-tests (plpgsql-parser)	1m 6s
parser-tests (parser)	58s
parser-tests (plpgsql-deparser)	1m 4s
parser-tests (traverse)	1m 2s
parser-tests (transform)	5m 38s
parser-tests (proto-parser)	56s
parser-tests (utils)	1m 3s
	13m 41s

Provide feedback