Update Airlock documentation

[rebkwok]

Update the Airlock docs to reflect the state of play at the end of stepping stone 3. This includes updating the overall releasing docs in the main docs repo to reference Airlock as the standard process (with a less high profile link to the old process).

[madwort]

was just documenting something else, so did a couple of drive-bys on the way past

#448
#449