link to `ggplot2::autoplot()` from all `autoplot` methods' documentation

[d-morrison]

update the documentation for:

• [ ] https://ucd-serg.github.io/serocalculator/reference/autoplot.curve_params.html
• [ ] https://ucd-serg.github.io/serocalculator/reference/autoplot.seroincidence.html

following the example in https://ucd-serg.github.io/serocalculator/reference/autoplot.pop_data.html