Build page structure in MDX instead of generating Markdown first

[Stvad]

This is an internal rendering change that should open way for things like:

• Make the lists collapsible (like in Roam) #23
• Roam "Document“ style missing #19
• Adding custom css classes on per-block basis
• Potentially also makes it easier to fix HTML elements on separate lines cause build to fail #21

[Stvad]

@tombarys @ghansali fyi pushed this out, this may impact CSS customizations used as it changes the structure of the DOM

• each block now has an id (that matches it’s roam block id)
• orphan blocks get id with orphan prefix and can be styled based on that
• html would be now be parsed on a best-effort basis (so you can do iframe/video embeds/etc) - before it was partially escaped/could break the build
• change to element structure (so css that relied on that closely may need to be changed)
• added additional wrappers around: block, block content, block-children, and those got corresponding classes