Move relevant docs from cbioportal to cbioportal-core

[forus]

Since cbioportal-core became a separate project, it makes sense to move the data loading documentation there as well.

Note: https://docs.cbioportal.org/ has to keep working

[inodb]

@forus that makes sense, were you thinking that the cbioportal docs site can pull the markdown from cbioportal-core? I think it could be done here:

https://github.com/cBioPortal/cbioportal/blob/master/.github/workflows/retype-action.yml

Might need to check out two repos or something