add xmind embed viewer

[thomasbreland]

• Uses the npm package recommended by our colleague: https://github.com/danielsss/embed-viewer
• In the future we could just directly use something like https://github.com/xmindltd/xmind-embed-viewer for more control of the formatting.
• I updated the xmind file to fold most of the subtopics - the viewer had trouble displaying it with all of the subtopics expanded - so you may wish to consider incrementing the file version number.
• To enable Pages for the respository: 'Settings' -> 'Pages' -> under 'Build and deployment' select 'GitHub Actions'
• You may wish to edit your repository details / about by checking 'Use your GitHub Pages website'

[thomasbreland]

Maybe "recommended" is too strong of a word. Suggested? Still open to alternatives, but it seems to work alright.

[thomasbreland]

I'm also noticing that it is sometimes difficult to fold subtopics once they are opened - may be worth addressing as a separate issue.