Fixed pkgdown configuration

[VisruthSK]

Summary

Fixes the pkgdown website. Closes #459.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if bba0f66 is merged into master:

• ✔️as_draws_array: 139ms -> 139ms [-0.47%, +0.48%]
• 🚀as_draws_df: 67.5ms -> 67ms [-1.52%, -0.02%]
• ✔️as_draws_list: 152ms -> 151ms [-0.78%, +0.11%]
• ✔️as_draws_matrix: 24.4ms -> 24.4ms [-0.82%, +0.63%]
• ✔️as_draws_rvars: 111ms -> 112ms [-0.79%, +0.86%]
• ✔️summarise_draws_100_variables: 727ms -> 727ms [-0.24%, +0.06%]
• ✔️summarise_draws_10_variables: 81.3ms -> 81.4ms [-0.2%, +0.33%]
  Further explanation regarding interpretation and methodology can be found in the documentation.