feat: add docs

docs/index.md

@@ -0,0 +1,16 @@
+# rules_bun docs
+
+Documentation site for `rules_bun`.
+
+## Rule reference
+
+- [rules.md](rules.md)
+
+## Regeneration
+
+The rule reference is generated from Starlark rule docstrings:
+
+```bash
+bazel build //docs:rules_md
+cp bazel-bin/docs/rules.md docs/rules.md
+```