The great refactor (#82)

source/plugins/pagespeed/README.md

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+
+### ⏱️ Website performances
+
+The *pagespeed* plugin adds the performance statistics of the website attached on your account:
+
+<table>
+  <td align="center">
+    <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.pagespeed.svg">
+    <details><summary>Detailed version</summary>
+      <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.pagespeed.detailed.svg">
+    </details>
+    <details><summary>With screenshot version</summary>
+      <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.pagespeed.screenshot.svg">
+    </details>
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+These metrics are computed through [Google's PageSpeed API](https://developers.google.com/speed/docs/insights/v5/get-started), which yields the same results as [web.dev](https://web.dev).
+
+See [performance scoring](https://web.dev/performance-scoring/) and [score calculator](https://googlechrome.github.io/lighthouse/scorecalc/) for more informations about how PageSpeed compute these statistics.
+
+Although not mandatory, you can generate an API key for PageSpeed API [here](https://developers.google.com/speed/docs/insights/v5/get-started) to avoid hitting rate limiter.
+
+Expect 10 to 30 seconds to generate the results.
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    plugin_pagespeed: yes
+    plugin_pagespeed_token: ${{ secrets.PAGESPEED_TOKEN }} # Optional but recommended
+    plugin_pagespeed_detailed: yes                         # Print detailed audit metrics
+    plugin_pagespeed_screenshot: no                        # Display a screenshot of your website
+    plugin_pagespeed_url: .user.website                    # Website to audit (defaults to your GitHub linked website)
+```