docs: first FURPs for Mix feature

draft-roadmap/introduce_mixnet_for_message_sending.md

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+# Introduce Mixnet For Message Sending
+
+**Estimated date of completion**: {Enter date}
+
+**Resources Required for 2025H2**:
+- 1 core research engineer for 3 months
+
+A PoC implementation to improve anonymity in Waku message publishing by mixing Waku Lightpush requests and responses.
+
+**FURPS** (see deliverables)
+
+**GitHub Milestone and deliverables**:
+
+## [Integrate libp2p mix into lightpush](https://github.com/waku-org/nwaku/issues/3280)
+
+**Owner**: core research
+
+**Feature**: [Mix](/FURPS/core/mix.md)
+
+**FURPS**:
+- F1. Relay nodes can mount mixnet protocol, acting as entry, exit or mixnet nodes
+- F2. Nodes can discover mixnet relay and exit nodes using existing peer discovery mechanisms
+- F3. Client nodes can send lightpush requests over the mixnet before delivery to a service node
+- F4. Client nodes can receive a response to a lightpush request over the mixnet
+
+**Checklist**:
+- [ ] Specs: link to specs
+- [ ] Code: link to GitHub issues/PRs/Epic
+- [ ] Dogfood: link to dogfooding session/artefact
+- [ ] Docs: links to README.md or docs.waku.org (TBD)