Include image of shape table

[teunbrand]

This PR aims to fix #4034.

It adds an svg image of the shape table to html docs and a pdf image of the shape table in the pdf docs.
I've checked with devtools::build_manual() that also the pdf documentation looks correct.

The code used for creating the figures is:

nms <- c(
  "square open", "circle open", "triangle open", "plus", "cross", 
  "diamond open", "triangle down open", "square cross", "asterisk", 
  "diamond plus", "circle plus", "star", "square plus", "circle cross", 
  "square triangle", "square", "circle small", 
  "triangle", "diamond", "circle", "bullet", "circle filled", "square filled", 
  "diamond filled", "triangle filled", "triangle down filled"
)
shape <- translate_shape_string(nms)

df <- data.frame(
  x = 0:25 %% 5,
  y = 0:25 %/% 5,
  name = nms,
  shape = shape
)

p <- ggplot(df, aes(x, y)) +
  geom_point(aes(shape = I(shape)), size = 4, fill = "red", stroke = 1) +
  geom_text(
    aes(label = paste0(shape, ":\n", name)),
    vjust = -0.5, size = 10, size.unit = "pt"
  ) +
  scale_y_continuous(transform = "reverse", limits = c(-0.5, 5.1)) +
  scale_x_continuous(limits = c(-0.5, 4.5)) +
  theme_void()

ggsave(
  file.path("man", "figures", "shape_table.svg"), 
  plot = p, device = svglite::svglite, fix_text_size = FALSE, 
  width = 6, height = 4, dpi = 72, bg = "white"
)
ggsave(
  file.path("man", "figures", "shape_table.pdf"), 
  plot = p,
  width = 6, height = 4, dpi = 72, bg = "white"
)