github-codespaces.md: Improve documentation

docs/github-codespaces.md

@@ -2,7 +2,20 @@
 
 A GitHub codespace container with all dependencies and several useful tools preinstalled is configured in the `.devcontainer` directory.
 
-You can go to https://github.dev/diasurgical/devilutionX/ and immediately begin developing.
+To develop in a codespace, open the in-browser VS Code editor by either pressing the `.` or going to https://github.dev/diasurgical/devilutionX/.
+
+Then, focus on the terminal window from the Command Palette menu (<kbd>F1</kbd>):
+
+![screenshot](gh-focus-on-terminal.png)
+
+Then, click "Continue Working in GitHub Codespaces":
+
+![screenshot](gh-continue-in-codespaces.png)
+
+It will take a few minutes to build the container.
+
+Once installed, the page will load back into the in-browser VS Code editor but now with a
+container shell.
 
 To view the graphical desktop of the container, click Ports > Browser.