docs: remove unused mammoth logo files and exclude the brand-assets README

Delete the placeholder mammoth logo and favicon, now unreferenced after wiring
the new logo. Exclude the open_pg_tde-logo/README.md reference file from the
built site so it does not appear as an orphan page.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: OffgridwithJD

documentation/docs/_images/mammoth-48.png

@@ -12,6 +12,10 @@ edit_uri: edit/main/documentation/docs
 
 use_directory_urls: false
 
+# Brand-asset folder READMEs are reference material, not site pages.
+exclude_docs: |
+  _images/open_pg_tde-logo/README.md
+
 # Theme settings
 theme:
   name: material