docs: Update docs for v5.1 release (#9958)

maintained-branches.md

@@ -2,7 +2,8 @@
 
 We are currently maintaining the following branches of Shaka Player:
 
- - v5.0 (latest)
+ - v5.1 (latest)
+ - v5.0 (previous)
  - v4.16 (LTS until January 31, 2027)
  - v4.15 (in use by the Cast Application Framework)
 

roadmap.md

@@ -15,11 +15,19 @@ Candidate features for future release cycles:
 v6.0
  - Conversion to Typescript (Smaller, more modular binary)
 
-v5.1 - 2026 Q2
+v5.2 - 2026 Q3
  - TBD
 
 =====
 
+v5.1 - 2026 Q2
+ - DASH: JSON manifest support (experimental build)
+ - ABR: Improved ABR with dropped frame monitoring
+   https://github.com/shaka-project/shaka-player/issues/745
+ - UI: Chapter images
+ - UI: Volume adjustment via mouse wheel
+ - Expanded platform support (TiVo OS, Titan OS)
+
 v5.0 - 2026 Q1
  - Add automatic subtitles with translations (Experimental and disabled by default)
    https://github.com/shaka-project/shaka-player/issues/9110