docs: add PostgreSQL 18 AST upgrade analysis #972

Job	Run time
parser-tests (deparser)	58s
parser-tests (proto-parser)	1m 2s
parser-tests (traverse)	53s
parser-tests (plpgsql-parser)	1m 2s
parser-tests (utils)	58s
parser-tests (pgsql-cli)	59s
parser-tests (transform)	5m 57s
parser-tests (plpgsql-deparser)	1m 8s
parser-tests (parser)	58s
	13m 55s

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