Adopt the openwashdata brand via washr::use_brand()

[larnsce]

Summary

• First data package on the central brand (openwashdata/brand): _brand.yml and the referenced logos installed by washr::use_brand(), _pkgdown.yml wired through template.bslib.brand, site rebuilt with Atkinson Hyperlegible and the brand palette.
• Pilot for the use_brand() rollout across the data package sites (use_brand(): install and refresh the openwashdata brand washr#110).

Test plan

• use_brand() with the default ref reports the brand up to date against openwashdata/brand main
• pkgdown::build_site() renders clean with the brand applied (requires the brand.yml package, declared in washr Suggests)
• (Visual) rebuilt site shows the brand typography and palette (owner reviewed the screenshot)