Each workspace crate's own README.md now also shows up as
docs/components/<crate>.md via nix/packages/reference-docs.nix, so it
renders on the docs site alongside the rest of docs/ without a second
hand-copied file to keep in sync. Relative links that were correct
from the crate's own position in the tree are rewritten for their new
virtual position (../docs/x.md -> ../x.md, ../<sibling-crate> ->
./<sibling-crate>.md). Landing page at docs/components/README.md, nav
bullet added to docs/README.md. No changes needed in the website
repo's docs.nix - it already walks every subdirectory generically.
