chore: ignore Obsidian vault and personal sidecar files

Add .obsidian/ (Obsidian vault config) and _*.md (personal sidecar
notes) to .gitignore so the docs/ tree can serve as canonical source
of truth while still being usable as an Obsidian vault for personal
authoring. The docs index generator already excludes the same _*.md
pattern from INDEX.md.

.gitignore

@@ -72,4 +72,8 @@ jp.sh
 
 # Lokale scratch-bestanden
 Brainstro
-/graphify-out
+/graphify-out
+
+# Personal Obsidian authoring layer (vault config + sidecar files prefixed `_`)
+.obsidian/
+_*.md