Remove locally-generated SNAPSHOT files from tracking

These files are generated by running Jekyll locally and should not
be committed. Add glob patterns to .gitignore to prevent recurrence.

Assisted-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>

Author: SamBarker

.gitignore

@@ -9,4 +9,7 @@ vendor
 _config-overrides.yml
 .ruby-version
 .DS_Store
-.op/
+.op/
+_data/documentation/*-SNAPSHOT.yaml
+_data/release/*-SNAPSHOT.yaml
+download/*-SNAPSHOT/