{Documentation} Core - Add 2.1.3 changelog entry

Summary:
WHY: 2.1.3 was released (D105974819 bumped version.txt) but the public-facing changelog on the docs site was not updated. Users browsing the Project Aria Tools doc site need to see what shipped in 2.1.3.

WHERE: docs-research-tools/projectariatools/changelog.mdx — the static doc site changelog page. Adds a new '## Version 2.1.3' section above 2.1.2 with the two changes from the 2.1.3 release: random-access P-frame decoding in getImageDataByIndex() for H.265 streams, and a VRS dependency bump to pick up ProximitySensorRecordableClass support.

HOW tested: Pure markdown / MDX content change, no code paths affected. Verified file renders by inspecting the diff against the existing 2.1.2 / 2.1.1 / 2.1.0 entries — same heading levels, same section structure (New Features / Improvements).

Reviewed By: PiotrBrzyski

Differential Revision: D106396894

fbshipit-source-id: b53ed1310d1aa989b40bb292f3f881d3fa4f2ce9

Author: kongchen1992

website/docs-research-tools/projectariatools/changelog.mdx

@@ -5,6 +5,17 @@ title: Changelog
 
 # Changelog
 
+---
+## Version 2.1.3
+
+### New Features
+
+- Added random-access P-frame video decoding: `getImageDataByIndex()` now supports non-sequential frame access for H.265 video streams. The decoder automatically replays from the preceding keyframe, enabling random-access for training data pipelines and video editors
+
+### Improvements
+
+- Bumped VRS dependency to include `ProximitySensorRecordableClass` support
+
 ---
 ## Version 2.1.2