plan: add documentation bug issues to BACKLOG.md

Author: krystophny

BACKLOG.md

@@ -1,6 +1,8 @@
 # Development Backlog
 
 ## TODO (Ordered by Priority)
+- [ ] #223: ASCII backend generates empty plots (only frames/borders)
+- [ ] #224: PDF files not deployed to GitHub Pages (404 errors)
 
 ## DOING (Current Work)
 - [x] #220: Fix ASCII backend generating PDF binary content instead of text output (branch: fix/comprehensive-ascii-cleaning)