Add function to list diseases included in database

[pratikunterwegs]

Is your feature request related to a problem? Please describe.
Participants in the first tutorial trial on early stage tasks consistently asked which diseases were included in the {epiparameter} database. While this information is available on the website it would be good to have it easily accessible via a function as well. This would help users more easily extract distributions for their disease of interest.

Describe the solution you'd like
A new function with a descriptive name that returns a data.frame with an approximation of the following: the formal disease name, the disease name listed in the database (e.g. "Ebola virus disease", "ebola"), the formal pathogen name, and the pathogen name listed in the database.

Alternatively, a section in the package vignettes with this information would achieve the same thing.

Additional context
None.

[joshwlambert]

Do the changes in PR #326 and the parameter_tbl() function (see README Quick Start section), in addition to the database vignette address the issue?

If not please specify which additional functionality would be useful.