Documenting available toolchains and toolchain versioning scheme

[mboisson]

We have settled on using 2020a toolchain versions from upstream whenever the components match upstream EasyBuild (and to try to make them map whenever possible). Variations of component versions will include the versions appended to 2020a (or other upstream versions in the future). We need to write documentation about this.

[mboisson]

Done in https://docs.google.com/spreadsheets/d/11kJfoZRjtsuPG06FgQyiE42513KGxVLqcu33-WGmMkA/edit#gid=0

[mboisson]

Also added to our internal documentation : https://wiki.computecanada.ca/staff/Using_EasyBuild_as_an_installer#Compute_Canada_toolchains