References to EPSG URL(s) are outdated in guidance

[nmtoken]

For example : in Vertical Extent https://agiorguk.github.io/gemini/datasets.html#_vertical_extent_information

or example as provided in the EPSG Geodetic Parameter Registry produced by The International Association of Oil & Gas Producers (see http://www.epsg-registry.org/)

Needs to change to https://epsg.org/

[nmtoken]

Also in services section https://agiorguk.github.io/gemini-dev/1063-gemini-services.html#16