Setting up plausible tracking

[kathryn-ods]

Overview

What does this pull request do?
Replaces the defunct google analytics tracking code with plausible tracking code

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Closes #

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• New or edited strings appearing as a result of this PR will be picked up for translation
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  translating. See the Handbook

Documentation & Release

• I've checked that the documentation builds without failing. See the Handbook
• I've thought about how and when this needs to be released. See the Handbook
• I've updated the changelog, if necessary. See the Handbook
• I've updated reference.rst to reflect any changes to schema structure or naming.
• I've added or edited any other related documentation. See the Handbook

[codemacabre]

Looks good to me!