expand vignettes

[lwaldron]

Good doc-a-thon items:

both vignettes:

• some introduction with background / rationale for the package and what it provides, linking to sources of more information
• citation & link to publication
• link to workshops

User Start Guide:

• why use this package? a little bit about cBioPortal and links / citations to more information about cBioPortal.
• what does it provide? (summarize of RE and MAE, with links to more information. Assume analyst comes here first without ever having heard of RE and MAE, and maybe not even (R)SE.
• How to know which datasets are available and working? Provide a link.
• how to choose whether to use the datapacks or the API?
• simple analysis examples
• how to request a dataset that's not importing, or bugs

API Reference:

• link to full API documentation
• limitations on data access (e.g. gene panels or few genes)