Migrate core publication to IG publisher

[vadi2]

This PR aims for a 1:1 translation from mkdocs to IG Publisher publication format. No parts of the specification will be changed, i.e. inputs and outputs will be exactly the same (typos and all).

Preview builds are available here.

Work done so far:

• transferred index.md as-is
• moved sections of spec out to separate pages
• switched to hl7 template
• updated sushi-config with the right properties
• migrated mkdocs-specific syntax, such as !!! note
• applied {:.grid} to all markdown tables to better reflect existing spec rendering in IG publisher view
• escaped {{ with {% raw %}{{{% endraw %} so it renders properly in markdown
• fixed italisation rendering by changing syntax from *_ to just _
• migrated OAuth 2.0 links
• removed <nobr> elements, they were breaking rendering and don't seem necessary in IG publisher
• fixed broken links due to page organisation

Out of scope for this PR:

• migrating each of the hooks to their own IGs

FYI:

• this IG shares the "no resources defined" warning in QA with FSH, as neither IGs have any resource profiles

[vadi2]

@isaacvetter @brynrhodes anything on this?