Update Amplitude Actions doc

[jenskene]

Fix instructions on how to find Amplitude API Key and Secret Key

Proposed changes

• Changed link so it goes to Amplitude workspace (seems it is not possible to deep link to a project if you don't know what the project is)
• Updated instructions to tell user to select the Project

Merge timing

• ASAP once approved

[jenskene]

Do we have a policy on whether external links - like the link to the Amplitude workspace - open in a new tab? Would be nice if it opened in a new tab!

[netlify]

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[markzegarelli]

Do we have a policy on whether external links - like the link to the Amplitude workspace - open in a new tab? Would be nice if it opened in a new tab!

@jenskene
No policy, but I agree that it would be great if external links opened in a new tab by default.

I just looked into it a bit, and by formatting the code as follows, the link will open in a new tab / window:

[Amplitude Workspace](https://analytics.amplitude.com){:target="_blank"}

I also updated the CSS to automatically add an external link icon to these links.