Add documentation for DevTools extensions (flutter#10061)

These docs will be linked to from the `devtools_options.yaml` file that
DevTools creates for storing extension enablement states:
flutter/devtools#7052

---------

Co-authored-by: Brett Morgan <brettmorgan@google.com>
Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>

src/_data/sidenav.yml

@@ -610,6 +610,8 @@
           permalink: /tools/devtools/logging
         - title: App size tool
           permalink: /tools/devtools/app-size
+        - title: DevTools extensions
+          permalink: /tools/devtools/extensions
         - title: Release notes
           permalink: /tools/devtools/release-notes
     - title: SDK overview

src/tools/devtools/extensions.md

@@ -0,0 +1,45 @@
+---
+title: DevTools extensions
+description: Learn how to use and build DevTools extensions.
+---
+
+## What are DevTools extensions?
+
+[DevTools extensions](https://pub.dev/packages/devtools_extensions) are developer
+tools provided by third-party packages that are tightly integrated into the
+DevTools tooling suite. Extensions are distributed as part of a pub package,
+and they are dynamically loaded into DevTools when a user is debugging their app.
+
+## Use a DevTools extension
+
+If your app depends on a package that provides a DevTools extension, the
+extension automatically shows up in a new tab when you open DevTools.
+
+### Configure extension enablement states
+
+You need to manually enable the extension before it loads for the first time.
+Make sure the extension is provided by a source you trust before enabling it.
+
+![Screenshot of extension enablement prompt]({{site.url}}/assets/images/docs/tools/devtools/extension_enable_prompt.png)
+
+Extension enablement states are stored in a `devtools_options.yaml` file in the
+root of the user's project (similar to `analysis_options.yaml`). This file
+stores per-project (or optionally, per user) settings for DevTools.
+
+If this file is **checked into source control**, the specified options are
+configured for the project. This means that anyone who pulls a project's
+source code and works on the project uses the same settings.
+
+If this file is **omitted from source control**, for example by adding
+`devtools_options.yaml` as an entry in the `.gitignore` file, then the specified
+options are configured separately for each user. Since each user or
+contributor to the project uses a local copy of the `devtools_options.yaml`
+file in this case, the specified options might differ between project contributors.
+
+## Build a DevTools extension
+
+For an in-depth guide on how to build a DevTools extension, check out 
+[Dart and Flutter DevTools extensions][article], a free article on Medium.
+
+
+[article]: {{site.flutter-medium}}/dart-flutter-devtools-extensions-c8bc1aaf8e5f