Enable site analytics via Plausible.io

SIG-Documentation would like enable site analytics and usage tracking for wasi.dev using the Bytecode Alliance's existing Plausible.io account (which we use today for bytecodealliance.org). To do that we need to embed the following script in the `` section of every page on the site, which for wasi.dev means properly via Docusaurus.

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I can handle the rest of the setup within our Plausible.io account once the script is inserted. Additional helpful information about the Plausible.io tracking snippet can be found [here](https://plausible.io/docs/plausible-script).