Add: document nasl grammar and predefined constants #1328

Job	Run time
audit	12s
clippy	2m 30s
formatting (nasl-syntax)	2s
formatting (storage)	3s
formatting (nasl-interpreter)	2s
typos	1m 13s
unittests (stable)	3m 29s
formatting (redis-storage)	2s
unittests (beta)	3m 35s
formatting (json-storage)	3s
unittests (nightly)	3m 40s
formatting (nasl-cli)	2s
rs-build-binaries / build	9m 41s
verify-feed-update	3m 31s
verify-syntax	1m 21s
	29m 26s

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