Rendering/Edit modes for guides

[tolusha]

We should use the editor area to display the guides directly. We will provide a switcher so that it's possible to go from one mode into the other one.

The basic requirements here is to provide a convenient editor for markdown/asciidoc files, which allow edition and preview.

By using tabs from the editor area to do that, there will be a lot of benefits:

User can split editor panes and organize the layout exactly the way he wants. He could display the guide on the left/right side by side with the code or other IDE elements.
User could open a guide in two different tabs, split the panes and display the two different modes (editing/rendering) side by side. This would make the edition and the turn around on the guides really convenient.
We don't invent any UX, we reuse what we already have in place and which users are easily understanding.
By default, we would open a guide in redering mode.

Epic: #6150

[che-bot]

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