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Fix sidebar package names and indentation in odoc-docsite

Add index.mld files for all packages that were missing them, which
caused odoc to generate default pages titled "<package> index". Also
fix leaf item indentation by always adding a spacer span regardless
of the node kind.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

+6
+2
doc/dune
··· 1 + (documentation 2 + (package x-ocaml))
+1
doc/index.mld
··· 1 + {0 OCaml notebooks as a WebComponent}
+2
doc/ppxlib_register/dune
··· 1 + (documentation 2 + (package ppxlib_register))
+1
doc/ppxlib_register/index.mld
··· 1 + {0 ppxlib registration helper}