Add Plausible for tracking metrics

[pavithraes]

Plausible (https://plausible.io/) is similar to Google Analytics, and we're using it on other Quansight project websites as well.

It'll help us track who visits our docs pages, and how often.

[trallard]

We can do this after #397

[trallard]

On hold until we merge #534

[kcpevey]

#534 has been merged so this is now unblocked.

[trallard]

@pavithraes shall we add Plausible to the existing conda?store docs? Or shall we wait until we have the new and updated docs?

[pavithraes]

I think we can wait for the new docs, but we can add Plausible once we have the initial set of pages ready