Still sandbox type @base seems required by index.md

[dgbroeder]

in index.md there is still mentioned a required preamble with a sandbox @base.
In #39 this was discussed in some length and all agreed that any @base that will ensure proper expansion of CURI type local_identifiers is permitted.

[rduyme]

Documentation about the @base included in https://github.com/skg-if/api

@andremann I think we should align
https://skg-if.github.io/interoperability-framework/ and https://github.com/skg-if/api about the base.