Add USD-EUR or EUR-USD currency conversion links

Description

In the handbook, the Sales Target Dinner Evangelism Reward section mentions the maximum amount of the incentive in USD but with a link to convert that to EUR (a Google search for "100 usd in eur"), like this:

The incentive is 100 USD to each team member […]

This is much more efficient and cuts down the time one has to manually make that conversion.

Proposal

I'm not sure if converting from USD to EUR (and vice-versa) is enough to cater all GitLabbers or if we need to support other currencies. Some options:

• Make this a standard in the handbook, linking all USD or EUR amounts to EUR or USD, respectively.
• Implement a tooltip that shows the converted amount. This maintains the reader in context and is faster to get the same info.
  • A variation could be to detect the local currency of the current browser location and show that in the tooltip