Update MyST reference instructions to use new UX (#43)

* Update MyST reference instructions to use new UX

Instead of URL editing, the user will have features in the user
interface to enable them to build and rebuild their MyST site.

This is a compromise, not our end goal; we want the skills they use in
the JupyterLab environment to be low-frication _and_ portable to their
local machine. I.e. we want them to use `myst start`. This is currently
not possible, and the MyST team expects it may take a month or more.

* Fix header level

Author: mfisher87

reference/04-using-myst.md

@@ -2,25 +2,29 @@
 
 ## Previewing your MyST site
 
-### First time
+### Building your site for the first time
 
-To view your built site:
+Every MyST site must have a `myst.yml` config file that defines the project.
 
-* Copy the URL of your server (e.g. Matt's URL is
-  `https://hub.cryointhecloud.com/user/mfisher87/lab`)
-* Open a new browser tab
-* Paste the URL in the URL bar
-* Replace `/lab` with `/myst-build/$DIRECTORY_OF_MYST_PROJECT`; for example, Matt's
-  project is at `~/workshop-open-source-geospatial`, so his URL would be
-  `https://hub.cryointhecloud.com/user/mfisher87/myst-build/workshop-open-source-geospatial`.
+Right-click this file in the file browser and select the first item on the context menu:
+"Build MyST Project".
 
-At this point you should see the build progress in your browser.
-It should only take a few seconds, and then you'll see your fully-built MyST site!
+A new tab will open displaying a loading progress indicator.
+After a few seconds, your site will be built and you'll it displayed in the current tab.
 
 
 ### Subsequent builds
 
-Your site will not automatically rebuild when you make changes!
-This functionality will be available in the future.
+In JupyterLab, your site will **not** automatically rebuild when you make changes!
+This functionality will be available in JupyterLab the future.
 
-Append  `/?rebuild=1` to the URL to trigger a rebuild.
+For now, the JupyterLab infrastructure will magically add a "Rebuild" button to the
+top-right of your site, which you can click to trigger a new build of your site.
+
+Do this every time you make a change to see if it worked correctly!
+
+
+### How this works
+
+This functionality is provided by the extension
+[`jupyter-myst-build-proxy`](https://github.com/ryanlovett/jupyter-myst-build-proxy/).