add uniformity_test to _pkgdown.yml

[florence-bockting]

Summary

The function uniformity_test is currently not listed in the _pkgdown.yml reference index.
Therefore pkgdown.yaml / pkgdown GitHub Action fails.

Change

• added uniformity_test to references section "Summarizing and diagnosing draws objects"
• running locally pkgdown::build_reference_index() passes now

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 817cf26 is merged into master:

• 🚀as_draws_array: 127ms -> 124ms [-2.63%, -2.13%]
• ✔️as_draws_df: 62.8ms -> 62.8ms [-0.53%, +0.59%]
• ✔️as_draws_list: 148ms -> 148ms [-0.27%, +0.42%]
• ✔️as_draws_matrix: 24.7ms -> 24.7ms [-0.49%, +0.12%]
• ✔️as_draws_rvars: 109ms -> 108ms [-1.73%, +0.47%]
• ✔️summarise_draws_100_variables: 719ms -> 719ms [-0.15%, +0.18%]
• ✔️summarise_draws_10_variables: 82.5ms -> 82.2ms [-0.62%, 0%]
  Further explanation regarding interpretation and methodology can be found in the documentation.