[repo] Document the process of upgrading Docusaurus

UPGRADING.md

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+# Upgrading Docusaurus
+
+The Moodle Developer resources is generated using an Open Source project, [Docusaurus](https://docusaurus.io/).
+
+Docusaurus periodically releases new versions which are announced on their website, and in the console when starting the Docusaurus development server.
+
+When new versions are released, we aim to update as soon as possible.
+
+## Process
+
+Prior to upgrading you should check the release notes for any potential issues. The full [changelog](https://docusaurus.io/changelog) links to the release notes for each version of Docusaurus.
+
+When starting the Docusaurus server, Yarn will provide instructions if there is a new version, for example:
+
+```
+╭──────────────────────────────────────────────────────────────────────────────────────╮
+│                                                                                      │
+│                    Update available 2.0.0-beta.18 → 2.0.0-beta.21                    │
+│                                                                                      │
+│      To upgrade Docusaurus packages with the latest version, run the following       │
+│                                      command:                                        │
+│         `yarn upgrade @docusaurus/core@latest @docusaurus/plugin-pwa@latest          │
+│      @docusaurus/preset-classic@latest @docusaurus/module-type-aliases@latest`       │
+│                                                                                      │
+╰──────────────────────────────────────────────────────────────────────────────────────╯
+```
+
+To upgrade the software, the provided command should be run:
+
+```
+yarn upgrade @docusaurus/core@latest @docusaurus/plugin-pwa@latest @docusaurus/preset-classic@latest @docusaurus/module-type-aliases@latest
+```
+
+After running the `yarn upgrade` you should check for any issues (especially those highlighted in the releases notes).
+
+Finally, you should run a `yarn clear && yarn build` to completely rebuild the documentation and check for errors.
+
+You should also look out specifically for infrastructure changes, such as a bump
+in the required NodeJS version (found in `.nvmrc` and `netlify.toml` files).