Working towards 508 Compliance

[brownag]

The HTML format of the bookdown book is reasonably good from aSection 508 Compliance perspective. It is easy to navigate with the keyboard (up/down to scroll, left/right arrow to change chapters, tab jumps to links/headers, works with screen readers, etc.

This issue is to identify tasks that we can perform to enhance/incrementally improve accessibility of the document.

• Add "alt" text to all images and figures that do not already have it (via markdown image syntax ![alttext](path/to/image.jpg), HTML alt tag, or knitr chunk option fig.caption
• Provide link to Rmd source of each HTML page and use knitr::purl() to provide an R script with code executed on each page
• Knit book to PDF via GitHub actions so link to download PDF (http://ncss-tech.github.io/stats_for_soil_survey/book/s4ssbook.pdf) works