Set visibility of markdown cells before rendering it

[mjbvz]

Backports fix for #126056 to 1.57

For #126056

This was regressed when we made rendering potentially be async

Before 1.57, rendering was always sync, so during initialization we would:

1. Create markdown cell
2. Render
3. Set visibility of the cell (usually to hidden)

For 1.57, we made rendering async. This can cause the following to happen:

1. Create markdown cell
2. await Render
3. A call to showMarkdownCell comes in and now reveal the cell
4. Render completes and we then incorrectly hide the markdown cell