[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.

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[Copilot]

Copilot review overview

🟡 Changes recommended

The sync workflow trusts mutable executable code with write permissions and its generated pull requests cannot trigger existing CI checks.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Mutable workflow ref enables unreviewed privileged code execution · New
• Generated PRs bypass validation due to default GITHUB_TOKEN · New

What changed in this PR

Adds the shared ForeSITE brand to the pkgdown site and establishes automated synchronization.

Changes:

• Adds canonical branding assets and pkgdown configuration.
• Adds required website dependencies and funding acknowledgement.
• Adds a weekly/manual brand-sync workflow.

File	Description
_pkgdown.yml	Enables branding, navbar, and footer.
DESCRIPTION	Declares the branding dependency.
NEWS.md	Records the rollout.
.github/​workflows/​pkgdown.yaml	Installs brand.yml.
.github/​workflows/​sync-foresite-brand.yml	Calls the reusable sync workflow.
pkgdown/​brand/​_brand.yml	Defines ForeSITE branding.
pkgdown/​brand/​assets/​logos/​foresite-black.png	Adds black logo.
pkgdown/​brand/​assets/​logos/​foresite-crimson.png	Adds crimson logo.
pkgdown/​brand/​assets/​logos/​foresite-gold.png	Adds gold logo.
pkgdown/​brand/​assets/​logos/​foresite-icon.png	Adds icon logo.
pkgdown/​brand/​assets/​logos/​foresite-primary.png	Adds primary logo.
pkgdown/​brand/​assets/​logos/​foresite-secondary.png	Adds secondary logo.
pkgdown/​brand/​assets/​logos/​foresite-white.png	Adds white logo.

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[gvegayon]

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