Exclude entire doc directory from distribution archive

Change `doc/grammars export-ignore` to `doc export-ignore` since the
doc directory only contains grammar files used for development and
does not need to be included in the distributed archive.

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

Author: 2 people

.gitattributes

@@ -4,7 +4,7 @@
 .github export-ignore
 apigen export-ignore
 phpcs.xml export-ignore
-doc/grammars export-ignore
+doc export-ignore
 tests export-ignore
 tmp export-ignore
 .editorconfig export-ignore