docs: add ReactiveX/rxdart to README and docs (#859)

https://github.com/ReactiveX/rxdart

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## Description

- add ReactiveX/rxdart to README

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [ ] ✨ `feat` -- New feature (non-breaking change which adds
functionality)
- [ ] 🛠️ `fix` -- Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ `!` -- Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 `refactor` -- Code refactor
- [ ] ✅ `ci` -- Build configuration change
- [x] 📝 `docs` -- Documentation
- [ ] 🗑️ `chore` -- Chore

Author: hoc081098

docs/index.mdx

@@ -69,6 +69,7 @@ The following projects are using Melos:
 - [leonardocustodio/polkadart](https://github.com/leonardocustodio/polkadart)
 - [powersync/powersync.dart](https://github.com/powersync-ja/powersync.dart)
 - [foss42/apidash](https://github.com/foss42/apidash)
+- [ReactiveX/rxdart](https://github.com/ReactiveX/rxdart)
 
 <Info>
 

packages/melos/README.md

@@ -210,6 +210,7 @@ The following projects are using Melos:
 - [powersync/powersync.dart](https://github.com/powersync-ja/powersync.dart)
 - [rodydavis/signals.dart](https://github.com/rodydavis/signals.dart)
 - [foss42/apidash](https://github.com/foss42/apidash)
+- [ReactiveX/rxdart](https://github.com/ReactiveX/rxdart)
 
 > Submit a PR if you'd like to add your project to the list. Update the
 > [README.md](https://github.com/invertase/melos/edit/main/packages/melos/README.md)