feat: add Doris brand assets page

[morningman]

Summary

• Add a new Brand Assets page for official Apache Doris SVG logo assets.
• Add 12 SVG assets under static/images/brand-assets with URL-friendly names.
• Add a Brand Assets link under the Resources footer section.

Test Plan

• Verified all asset file names referenced by the page exist under static/images/brand-assets.
• Ran git diff --check on the changed files.
• Full build not run in this session; user will compile locally.