Folgezettel-aware sidebar + new configurable 'default' layout

[srid]

Resolves #473 by making the sidebar tree be built based on not only the folder structure but also folgezettel relationships.

Also replaces both 'book' and 'note' layout with a single 'default' layout in which all of sidebar, uptree and breadcrumbs can be toggled on or of.

Still exploring this feature ...

• Create a working version for nixos.asia
• [ ] A notion of 'primary' parent?
  • Breadcrumbs still use folder structure (see routeBreadcrumbs); what to use? (Answer: fine as it is for now)
  • [ ] Do we trim excessive parents? https://mathworld.wolfram.com/MaximumSpanningTree.html
• Change semantics of folder-folgezettel (reverse). Done ... does it make sense?
• "Index" (-/all) should always include all notes, even if they cannot be reached from top (due to folder-folgezettel being cut off at some point or due to disconnected graphs). Perhaps this can be implemented by having index folder folgezettel automatically include orphans somehow (if so, under what user config?) https://en.wikipedia.org/wiki/Spanning_tree
  • Fix excessive use of bold in Index
  • Avoid having to specify folder-folgezettel to false in index note (for that private notebook deployed to CF pages)
• use a different svg icon than 'folder'?
• Replace 'book' and 'note' layout with a single layout
• Performance (memoize tree)
• Refactor

[srid]

First pass, showing multiple parents. Obviously the sidebar needs to be trimmed:

[srid]

Using this in https://nixos.asia/en now as of nixos-asia/website@90db854. en.md specifies the top-level folgezettel relationship explicitly, so now our sidebar looks:

[srid]

It is a bit jarring to see flake-parts appear (unnecessarily) in two sub-trees here: https://nixos.asia/en/treefmt