use_brand(): install and refresh the openwashdata brand from openwashdata/brand

[larnsce]

The canonical brand now lives in openwashdata/brand (_brand.yml plus the logo library; see openwashdata/brand#1). Its README promises a washr helper to automate the copy into consuming repos. This issue is that helper.

Proposed behavior, following the existing washr verb conventions:

• use_brand() copies the current _brand.yml from openwashdata/brand (main) into the project root, plus the logo files the brand file references, adjusting logo paths for the destination layout.
• Re-running refreshes an existing copy to the latest version and messages what changed; the function is idempotent.
• An option (or a companion step in setup_website()) applies the brand to the pkgdown site via bslib's brand support, so data package sites finally carry the visual identity. Today washr's _pkgdown.yml template ships structure only (Plausible, funding sidebar, authors footer) and the roughly 30 package sites run stock Bootstrap 5 with none of the brand fonts or colors.
• Distribution discipline stays change-here-first: values change in openwashdata/brand, consumers refresh via use_brand(); the function never edits brand values locally.

Acceptance criteria:

• use_brand() installs _brand.yml and referenced logos into a fresh package and messages the files written
• a second run against an updated brand repo refreshes the files and reports the change
• the pkgdown integration renders a package site with Atkinson Hyperlegible and the brand palette
• documented in the README alongside the other setup functions

Fits the v1.2.0 scope discussion. Refs the metadata decision doc conventions from #67 for where the design note lives.