Codespaces: Add info on linking to existing codespaces (#40291)

Co-authored-by: Ben Ahmady <32935794+subatoi@users.noreply.github.com>

content/codespaces/developing-in-codespaces/creating-a-codespace-for-a-repository.md

@@ -168,3 +168,4 @@ For full details of the options for this command, see [the {% data variables.pro
 
 - "[AUTOTITLE](/codespaces/developing-in-codespaces/opening-an-existing-codespace)"
 - "[AUTOTITLE](/codespaces/setting-up-your-project-for-codespaces/setting-up-your-repository/facilitating-quick-creation-and-resumption-of-codespaces)"
+- "[AUTOTITLE](/rest/codespaces)" (REST API reference)

content/codespaces/developing-in-codespaces/opening-an-existing-codespace.md

@@ -75,6 +75,8 @@ You can bookmark the address of this page if you want to get back to it quickly
 
    If you choose **JupyterLab**, the JupyterLab application must be installed in the codespace. {% data reusables.codespaces.jupyterlab-in-default-image %}
 
+{% data reusables.codespaces.linking-to-an-existing-codespace %}
+
 {% endwebui %}
 
 {% vscode %}
@@ -100,6 +102,9 @@ You can also access the commands listed above by navigating to the Remote Explor
 ![Screenshot of a codespace selected in the Remote Explorer, with "Open in Browser" highlighted in the right-click menu.](/assets/images/help/codespaces/open-codespace-remote-explorer.png)
 
 {% data reusables.codespaces.remote-explorer %}
+
+{% data reusables.codespaces.linking-to-an-existing-codespace %}
+
 {% endvscode %}
 
 {% cli %}
@@ -159,3 +164,7 @@ For more information, see [`gh codespace code`](https://cli.github.com/manual/gh
 {% data reusables.codespaces.opening-codespace-in-jetbrains %}
 
 {% endjetbrains %}
+
+## Further reading
+
+- "[AUTOTITLE](/rest/codespaces)" (REST API reference)