Mark rendered SEP docs as linguist-generated

The `render-seps.ts` script generates `docs/community/seps/index.mdx`
and individual SEP pages like `docs/community/seps/1850-*.mdx`. Mark
these with `linguist-generated=true` so GitHub collapses them in diffs.
The `[0-9]*` pattern avoids accidentally matching non-generated files
that don't start with a digit.

Author: jonathanhefner

.gitattributes

@@ -2,3 +2,5 @@ package-lock.json linguist-generated=true
 schema/*/schema.json linguist-generated=true
 docs/specification/*/schema.md linguist-generated=true
 docs/specification/*/schema.mdx linguist-generated=true
+docs/community/seps/index.mdx linguist-generated=true
+docs/community/seps/[0-9]*.mdx linguist-generated=true