The key folder where the AsciiDoc specification is developed is GeoTIFF_Standard cf. https://github.com/opengeospatial/geotiff/tree/master/GeoTIFF_Standard/standard Other files of interest are:
Currently we are working towards the version 1.2
In addition, you can find the AsciiDoc version of the SWG charter https://github.com/opengeospatial/geotiff/tree/master/GeoTIFF_charter