feat: add plugin system and experimentation plugin to the site

As discussed with @trieloff during our recent workshop, I'm adding the plugin system and the experimentation plugin to the website.

## Description

1. Adds the minimal plugin system proposal from adobe/aem-boilerplate#254
2. Adds the experimentation plugin from https://github.com/adobe/aem-experience-decisioning/tree/plugin-api
3. Instrument the existing `performance.js` as a plugin

.eslintignore

@@ -1,2 +1,3 @@
 tools/sidekick
 libs
+plugins/

.stylelintignore

@@ -1,2 +1,3 @@
 /libs/highlight/*.css
-/tools/sidekick
+/tools/sidekick
+plugins/

plugins/experimentation/.eslintignore

@@ -0,0 +1 @@
+src/ued.js

plugins/experimentation/.eslintrc.js

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+module.exports = {
+  root: true,
+  extends: 'airbnb-base',
+  env: {
+    browser: true,
+  },
+  parser: '@babel/eslint-parser',
+  parserOptions: {
+    allowImportExportEverywhere: true,
+    sourceType: 'module',
+    requireConfigFile: false,
+  },
+  rules: {
+    // allow reassigning param
+    'no-param-reassign': [2, { props: false }],
+    'linebreak-style': ['error', 'unix'],
+    'import/extensions': ['error', {
+      js: 'always',
+    }],
+  },
+};

plugins/experimentation/.stylelintrc.json

@@ -0,0 +1,8 @@
+{
+  "extends": [
+    "stylelint-config-standard"
+  ],
+  "rules": {
+    "declaration-block-no-redundant-longhand-properties": null
+  }
+}

plugins/experimentation/CODE_OF_CONDUCT.md

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+# Adobe Code of Conduct
+
+## Our Pledge
+
+In the interest of fostering an open and welcoming environment, we as
+contributors and maintainers pledge to making participation in our project and
+our community a harassment-free experience for everyone, regardless of age, body
+size, disability, ethnicity, gender identity and expression, level of experience,
+nationality, personal appearance, race, religion, or sexual identity and
+orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to creating a positive environment
+include:
+
+* Using welcoming and inclusive language
+* Being respectful of differing viewpoints and experiences
+* Gracefully accepting constructive criticism
+* Focusing on what is best for the community
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+Examples of unacceptable behavior by participants include:
+
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plugins/experimentation/LICENSE.md

@@ -0,0 +1,201 @@
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