docs: Add Upcoming section to changelog with teletext multi-page feature

Start new changelog section for unreleased changes. First entry is
the multi-page teletext extraction feature (#665) which allows
extracting multiple teletext pages simultaneously with separate
output files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

docs/CHANGES.TXT

@@ -1,3 +1,10 @@
+Upcoming (unreleased)
+---------------------
+- New: Multi-page teletext extraction support (#665)
+  - Extract multiple teletext pages simultaneously with separate output files
+  - Use --tpage multiple times (e.g., --tpage 100 --tpage 200)
+  - Output files are named with page suffix (e.g., output_p100.srt, output_p200.srt)
+
 0.96 (2025-12-21)
 -----------------
 - New: Added --list-tracks (-L) option to list all tracks in media files without processing