Add demo section to local unpartitioned access explainer. (#208)

* Add demo section to local unpartitioned data access explainer.

This section adds the command line flags needed in Chromium to test the unpartitioned data access feature, and links to the demo sites we made for TPAC. This will make it easy for interested folks to test the personalized payment button use case on Chrome Canary.

* Fix link formatting.

explainer/fenced_frames_with_local_unpartitioned_data_access.md

@@ -292,6 +292,17 @@ function personalizeButton () {
 }
 ```
 
+## Try it yourself!
+
+Development of this feature is ongoing, but the core functionality described above is implemented in Chromium and can be enabled with two command line flags. **We recommend using Chrome Canary to test with our most recent changes.**
+
+* `--enable-features=FencedFramesDefaultMode,FencedFramesLocalUnpartitionedDataAccess`    
+* `--disable-features=EnforcePrivacySandboxAttestations`
+
+We also have some Glitch demo sites for testing the personalized payment button use case.
+
+* First, visit [demo-payments-provider.glitch.me](https://demo-payments-provider.glitch.me) to register credit card info (please don't use a real credit card number 🙂).
+* Then, visit [demo-merchant.glitch.me](https://demo-merchant.glitch.me) for a sample checkout experience with a personalized payment button.
 
 ## Privacy considerations
 
@@ -375,8 +386,9 @@ The benefits of using cookies for this solution are the following:
 *   Local storage, like other storage APIs, is being [partitioned](https://privacycg.github.io/storage-partitioning/) in iframes. To allow unpartitioned access, it will rely on a variant of [rSA for storage](https://groups.google.com/a/chromium.org/g/blink-dev/c/SEL7N-xIE5s). 
 *   Fenced frames, by default have a unique and ephemeral partition and either we would disable that access in read-only FFs or there would be a transition where the value of a given data could be different before and after the access is granted, leading to confusion.
 
-
 ## References
 [TPAC presentation from Sep 2023](https://docs.google.com/presentation/d/1TqtFtK4x3TMd96JEvkbApUaYVdIaUz9uz3wNGPTuqdU/edit?usp=sharing)
 
+[TPAC presentation from Sep 2024](https://docs.google.com/presentation/d/1xKwCrhN2pD_lxuPfurgjEDkfyAcf3wgxPS9njlG9tT0/edit?usp=sharing)
+
 [Related Issue](https://github.com/WICG/fenced-frame/issues/15)