Add fig.alt to vignette figures (a11y / pkgdown warnings)

[adibender]

Building pkgdown surfaces several Missing alt-text accessibility warnings across the vignettes:

• vignettes/baseline.Rmd
• vignettes/basics.Rmd
• vignettes/competing-risks.Rmd
• vignettes/convenience.Rmd
• vignettes/cumulative-effects.Rmd
• vignettes/data-transformation.Rmd
• vignettes/frailty.Rmd
• vignettes/left-truncation.Rmd
• vignettes/model-evaluation.Rmd
• vignettes/multi-state.Rmd
• vignettes/recurrent-events.Rmd
• vignettes/splines.Rmd
• vignettes/strata.Rmd

Sweep PR: add fig.alt = "..." to plotting chunks. See vignette(pkgdown::accessibility).