Add frequencies panel to existing pathogens

[genehack]

From the priorities doc "somebody should" section:

Add frequencies panel to a bunch of our existing pathogens, most relevant for dengue, mpox, rabies, RSV (especially), seasonal CoVs, avian flu

Additional Slack context from @huddlej:

ncov and seasonal-flu are the two workflows I’m aware of that support frequencies. The tip frequencies rule in ncov shows how we tend to run the augur frequencies command with the KDE method to produce the tip frequencies JSON. The default parameters for ncov show one approach to parameterizing frequencies. This ancient PR and its corresponding docs build are the only “public” docs about the KDE frequency method.

TODO list:

• dengue
• mpox
• rabies
• RSV
• Add frequencies panel seasonal-cov#41
• avian flu

N.b., if you decide to do this work, your first step should be creating an issue in the repo you'll be working in, link that issue back to here, and update the above list to point to the specific repo's issue