Document common conversion pairs

[MantisClone]

Problem

We don't have any documentation page that lists all the supported conversion pairs.

Details

According to @yomarion, we support many pairs and it depends on the chain. The RF currency API caches possible conversions and could be a good starting place. If you can't find the endpoint, check the network tab on the Request Finance invoice creation page load, you should find it.

Priority

Develop this after completing #15

[yomarion]

@MantisClone this is the endpoint I was referring to: https://api.request.finance/currency/aggregators

[MantisClone]

We can use the Price Aggregator subgraphs to query the full list of conversion pairs on each chain.