docs: add Roadmap

[p-shahi]

@achingbrain please feel free to push edits directly to this branch

relates to:

• feat: circuit v2 #1166
• feat: add autonat support #1298
• Track: enabling the DHT by default ipfs/js-ipfs#3905

consider pulling in:

• Consider libp2p mono repo #824
• Libp2p testing overhaul #857
• Connection Manager Overhaul #744

[BigLep]

Similar to comment elsewhere:

1. Add a heading about the status
2. Make it clear how someone follows along with this as it evolves and how they leave comments/feedback
3. Link to this from other places (e.g., README, libp2p/specs vision ROADMAP.md).

[BigLep]

Thanks for incorporating feedback @p-shahi and @achingbrain !

[achingbrain]

Some great stuff here, I'm happy for this to be merged once all the comments above are addressed.

[p-shahi]

@achingbrain thanks, added a section on "Ergonomic metrics API" and moved the Browser<->Browser to Mid Q2.

If the estimated timeline needs to be shuffled, let me know what to correct

[p-shahi]

@BigLep I think Alex is on PTO, can you take a look to see if we are aligned on the timeline

Specifically this portion

#### End of Q4 (December)
- [A.2 📺 WebRTC: Browser to Server](#2-webrtc-browser-to-server)
- [B.1 🥊 AutoNat](#1-autonat)
- [B.2 🥊 Circuit Relay v2](#2-circuit-relay-v2)

### 2023

#### Early Q1 (January)
- [C.1 🔮 Ergonomic metrics API](#1-ergonomic-metrics-api)
- [***➡️ test-plans/DHT server mode scale test***](https://github.com/libp2p/test-plans/blob/master/ROADMAP.md##1-dht-server-mode-scale-test)

#### Mid Q1 (February)
- [A.3 📺 WebRTC: Browser to Browser](#3-webrtc-browser-to-browser)

#### End of Q1 (March)
- [B.3 🥊 Add QUIC Transport](#3-add-quic-transport)
- [***➡️ test-plans/Benchmarking using remote runners***](https://github.com/libp2p/test-plans/blob/master/ROADMAP.md#2-benchmarking-using-remote-runners)

C.1 is early Jan (maybe we can get to it sooner but hard to say) because it would be a big QoL improvement for @dapplion and Lodestar, perhaps we can get to it sooner than that.

A.3 is Mid Q1 but I might be underestimating the complexity here. Wdyt?

B.3 is there as a hopeful target but dependent on nodejs/node#44325
(there is recent activity/momentum in that PR though!)

[BigLep]

Thanks @p-shahi . Looks good!