docs: update FAQ with extension gallery info (#2672)

Author: Akash Satheesan

docs/FAQ.md

@@ -101,8 +101,11 @@ Service](https://cdn.vsassets.io/v/M146_20190123.39/_content/Microsoft-Visual-St
 > Visual Studio Products and Services.
 
 Because of this, we can't offer any extensions on Microsoft's marketplace.
-Instead, we've created a marketplace offering open-source extensions. The
-marketplace works by scraping GitHub for VS Code extensions and building them.
+Instead, we use the [Open-VSX extension gallery](https://open-vsx.org), which is also used by various other forks.
+It isn't perfect, but its getting better by the day with more and more extensions.
+
+We also offer our own marketplace for open source extensions, but plan to
+deprecate it at a future date and completely migrate to Open-VSX.
 
 These are the closed-source extensions that are presently unavailable:
 
@@ -118,15 +121,8 @@ For more about the closed source portions of VS Code, see [vscodium/vscodium](ht
 
 ## How can I request an extension that's missing from the marketplace?
 
-We are in the process of transitioning to [Open VSX](https://open-vsx.org/).
-Once we've [implemented Open
-VSX](https://github.com/eclipse/openvsx/issues/249), we can finalize this
-transition. As such, we are not currently accepting new extension requests.
-
-In the meantime, we suggest:
-
-- [Switching to Open VSX](#how-do-i-configure-the-marketplace-url) now
-- Downloading and [installing the extension manually](#installing-an-extension-manually)
+To add an extension to Open-VSX, please see [open-vsx/publish-extensions](https://github.com/open-vsx/publish-extensions).
+We no longer plan to add new extensions to our legacy extension gallery.
 
 ## How do I install an extension?
 
@@ -159,20 +155,19 @@ You can also download extensions using the command line. For instance,
 downloading from OpenVSX can be done like this:
 
 ```shell
-SERVICE_URL=https://open-vsx.org/vscode/gallery ITEM_URL=https://open-vsx.org/vscode/item code-server --install-extension <extension id>
+code-server --install-extension <extension id>
 ```
 
 ## How do I use my own extensions marketplace?
 
 If you own a marketplace that implements the VS Code Extension Gallery API, you
-can point code-server to it by setting `$SERVICE_URL` and `$ITEM_URL`. These correspond directly
-to `serviceUrl` and `itemUrl` in VS Code's `product.json`.
+can point code-server to it by setting `$EXTENSIONS_GALLERY`.
+This corresponds directly with the `extensionsGallery` entry in in VS Code's `product.json`.
 
-For example, to use [open-vsx.org](https://open-vsx.org), run:
+For example, to use the legacy Coder extensions marketplace:
 
 ```bash
-export SERVICE_URL=https://open-vsx.org/vscode/gallery
-export ITEM_URL=https://open-vsx.org/vscode/item
+export EXTENSIONS_GALLERY='{"serviceUrl": "https://extensions.coder.com/api"}'
 ```
 
 Though you can technically use Microsoft's marketplace in this manner, we