Add navigation and authentication vignettes

[garrettgman]

Add an extended version of https://github.com/rstudio/webinars/blob/master/32-Web-Scraping/navigation-and-authentication.md as a vignette that explains navigation and authentication/form submission.

[petrbouchal]

Just in case this comes in useful, I have written a walk-through of using part of the navigation workflow in rvest 1.0.0 on a particularly tricky website https://petrbouchal.xyz/post/rvest-time/

[AmeliaMN]

@petrbouchal thank you for your post! It gave me the confidence to be able to solve a similar two-step authentication that had been tripping me up. Details on RStudio Community if you are interested.

[petrbouchal]

@petrbouchal thank you for your post! It gave me the confidence to be able to solve a similar two-step authentication that had been tripping me up. Details on RStudio Community if you are interested.

Great to hear that it was helpful!