docs: add scully-plugin-canonical (#1167)

Added the scully-plugin-canonical information to the community plugin list

Author: pjlamb12

docs/Reference/plugins/community-plugins/canonical.md

@@ -0,0 +1,89 @@
+---
+title: Canonical Plugin
+published: true
+lang: en
+position: 100
+---
+
+# `rss` Plugin
+
+<div class="docs-link_table">
+  <a class="homepage" href="https://github.com/pjlamb12/scully-plugin-canonical"></a>
+  <a class="repository" href="https://github.com/pjlamb12/scully-plugin-canonical"></a>
+</div>
+
+## Overview
+
+The purpose of the Scully canonical plugin is to take a canonical link attribute from a route and replace the value of the canonical link in the head of a page, or add the link tag if it doesn't exist. This is needed for SEO purposes, if a given article was first published on another site, for example.
+
+## Features
+
+- ✅ Replaces the canonical link with the provided URL for the link tag
+- ✅ Adds a canonical link tag if one doesn't exist for a page
+
+## Table of Contents
+
+- [Installation](#installation)
+- [Usage](#usage)
+- [FAQ](#faq)
+
+## Installation
+
+### NPM
+
+`npm install scully-plugin-canonical --save-dev`
+
+### Yarn
+
+`yarn add scully-plugin-canonical --dev`
+
+## Usage
+
+Render plugins get access to information about the route, and the plugin looks for the title in the following locations in order:
+
+- `route.canonical`
+- `route.data.canonical`
+- `route.canonicalUrl`
+- `route.data.canonicalUrl`
+- `route.canonical_url`
+- `route.data.canonical_url`
+
+> If the canonical url is set in the frontmatter of a markdown file, the attribute shows up on `route.data`.
+
+To use this plugin, you need to just require the package inside the Scully `config.ts` for your project, i.e. `scully.your-project-name.config.ts`. After requiring the plugin, add it to the `defaultPostRenderers` array for the site:
+
+```ts
+// scully.your-project-name.config.ts
+require('scully-plugin-canonical');
+
+export const config: ScullyConfig = {
+  projectRoot: './src',
+  projectName: 'your-project-name',
+  outDir: './dist/static',
+  routes: {},
+  defaultPostRenderers: ['setCanonicalLinkPlugin'],
+};
+```
+
+If you only want it to be added to some routes, add it like this:
+
+```ts
+// scully.your-project-name.config.ts
+require('scully-plugin-canonical');
+
+export const config: ScullyConfig = {
+  ...
+  routes: {
+    '/blog/:slug': {
+      type: 'contentFolder',
+      slug: {
+        folder: './blog'
+      },
+      postRenderers: ['setCanonicalLinkPlugin']
+    }
+  }
+  ...
+};
+```
+
+That's all it takes for the plugin to be included and run on the pages in your app.