Soft deprecate `add_figure()`

[kellijohnson-NOAA]

https://github.com/nwfsc-assess/sa4ss/blob/b8b66eb0964cd80ce40d4b9820123e0b4a93d728/R/add_figure.R#L55-L62

is no longer needed because {rmarkdown} has come a long way since 2020 when alternative text was not standard and could not be added via the code chunk options. Starting about April of 2021, {knitr} started allowing `fig.alt' as a chunk option (see the Rstudio blog post about it. So, add your figures however you desire, just be sure to include alternative text for those that need it. 🏆 for those in need of alternative text ♿.

• [ ] create a warning using {lifecycle} warning users that they no longer need to use add_figure(), i.e., soft deprecate
• [ ] provide an example of how to add a figure
• [ ] check that add_figure() is removed from other files in the code

[kellijohnson-NOAA]

I am no longer convinced that we don't need this. I have found a way to tag the document and use alternative text with the newest version of LaTeX but it requires extra text and some standard formatting. I am not sure if we will be able to do this in the .Rmd file as my only experience with a successful build is from the .tex file. We might have to use grep or sed to alter the built .tex file.

[kellijohnson-NOAA]

Closing this issue because it might be easier to know that all figures are imported in a standard way to keep up with the latest changes of LaTeX.