Add rrweb as open-source alternative to FullStory

Using CONTRIBUTING_TO_OSB.md, please add:

Add proprietary tool: FullStory
- Description: Session replay and digital experience analytics platform
- Website: https://www.fullstory.com
- Category: Analytics
- Features: Session replay, Heatmaps, Frustration signals, Funnels and conversions, Mobile app replay

Open source alternatives:
1. rrweb - https://github.com/rrweb-io/rrweb

rrweb records the DOM and user interactions as a typed JSON event stream and replays them pixel-perfect. MIT licensed, TypeScript, 19,899 GitHub stars. Website: https://rrweb.com, docs: https://rrweb.com/docs/. It is the recording engine behind the session replay features of Sentry, PostHog, Amplitude, and Highlight, and covers the session replay use case of Hotjar and LogRocket as well.

Disclosure: I work on rrweb.

Read CONTRIBUTING_TO_OSB.md first, then locate the existing FullStory entry and its Analytics category. Add the requested FullStory details and list rrweb as an open-source alternative with its supplied links and description. Done means the entry follows the repository's formatting and includes the disclosure.

Written by the indexing model from the issue text.