task: Make sure every end-user project listed in the docs is also on celo.org/ecosystem

Part of #2266.

### What needs to be done?

The docs list end-user projects that belong on the ecosystem site as well. At `bdf40b37`:

- `home/wallets.mdx` — 13 wallet entries
- `home/exchanges.mdx` — 7 exchanges (Binance, Coinbase, Uniswap, Velodrome, Ubeswap, Mento, plus a CoinMarketCap link)
- `home/ramps.mdx` — 20 ramp providers (Banxa … Yellow Card)

Decided: the docs stay the developer-facing source, and end-user discovery of these projects belongs on https://celo.org/ecosystem (verified 200). Long-term the docs should be able to point there instead of maintaining vendor lists.

### Definition of done

- [ ] Sheet: every project from the three pages × present on celo.org/ecosystem yes/no (link in this issue)
- [ ] Ops (outside this repo): missing projects submitted to the ecosystem-site owner through their intake process
- [ ] Once listed, `home/wallets`, `home/exchanges`, `home/ramps` (or their Learn successors after #2258) link to the ecosystem page as the canonical end-user directory
- [ ] Decision recorded (OPEN): keep the vendor lists in the docs as well, or reduce them to "see the ecosystem page" once parity is confirmed

### Non-goals

Splitting end-user vs developer wallet pages (#2256); moving pages (#2258).

### Depends on / blocks

No dependencies — can start now. #2258 must not delete these pages before this lands.

**Measured at:** `bdf40b37`