ENH: Update extension metadata

[jcfr]

Consolidate extension metadata based on the corresponding s4ext file organized
in the ExtensionsIndex repository.

[gattia]

Hey, looks like the icon path is moved to a location that doesn't exist. Is this correct? Im not totally sure on what the suggested tests are to make sure this is working.

[jcfr]

The EXTENSION_ICONURL variable is set to the "raw" URL. This table should help illustrate:

	URL
GitHub UI URL	https://github.com/gattia/Slicer-T2mapping/blob/master/T2mapping/Resources/Icons/T2mapping.png
GitHub Raw URL	https://raw.githubusercontent.com/gattia/Slicer-T2mapping/master/T2mapping/Resources/Icons/T2mapping.png

Reference:

• https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#viewing-or-copying-the-raw-file-content

[jcfr]

We can also confirm the raw link can render as an image:

Markdown	Rendered Markdown