[codex] Add ForeSITE pkgdown brand

[gvegayon]

Summary

Related to EpiForeSITE/rbranding#77. This is a downstream rollout of the
canonical ForeSITE pkgdown brand.

The package had no local copy of the shared brand, so its documentation site
could not inherit the ForeSITE colors, typography, navigation treatment, logo
rules, or standard funding acknowledgement. This PR copies the canonical
_brand.yml and seven logo assets into pkgdown/brand/, points pkgdown at the
brand, and adds the shared navbar and footer settings while preserving the
package-specific site configuration.

It also declares brand.yml as a website dependency, installs it in the
pkgdown workflow, records the rollout in NEWS, and adds a weekly/manual caller
for the reusable brand-sync workflow. The caller follows main; it will open a
package-local pull request whenever the canonical brand changes. The reusable
workflow is supplied by EpiForeSITE/rbranding#79 and should merge first.

Validation

• Parsed the pkgdown and GitHub Actions YAML files.
• Confirmed the complete canonical logo set and brand file are present.
• Ran git diff --check.
• Built the pilot package sites for facilityepimath and linkeR successfully;
  their pkgdown metadata reported only pre-existing URL warnings.

[gvegayon]

@damontoth, this is 100% branding, which does not affect package functionality. I will go ahead and merge this.