docs: add rstudio docs

[wolfv]

Just a few docs on how to use pixi + RStudio.

[ruben-arts]

It would be awesome to have a little example of a script. As it currently doesn't work for me. Gary generated this for me:

# Load the necessary library
library(ggplot2)

# Create some example data
data <- data.frame(
  x = rnorm(100),  # 100 random numbers (normal distribution)
  y = rnorm(100)
)

# Create a scatter plot
plot <- ggplot2(data, aes(x=x, y=y)) +  # aes defines aesthetic mappings
  geom_point() +  # geom_point() for scatter plot
  ggtitle("Example Scatter Plot") +  # Add a title
  theme_minimal()  # Use a minimal theme

# Display the plot
print(plot)

But it tells me it doesn't have the ggplot2 lib.

Also not having rstudio-desktop in the toml is a bit anti pixi. Should we add it to the dependencies?