Support custom corpus documentation templates

Mount a directory with Kalamar templates into the kalamar container, so
users can describe their own corpus in the KorAP user interface instead
of the placeholder page Kalamar shows by default. It defaults to
./custom and is created on demand; KALAMAR_TEMPLATES points it
elsewhere.

The mount itself is part of KorAP-Docker's compose file, whose
custom/README.md documents the available templates and ships examples;
this project only passes the directory through.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: I7a1250020c2625b38dee2c1e4c33508f5af1eb8e
diff --git a/.gitignore b/.gitignore
index a2efa56..0ed2fda 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,3 +33,7 @@
 
 # Log files
 *.log
+
+# Kalamar templates describing your corpus (created on demand by `make korap`,
+# see KALAMAR_TEMPLATES). Track them in your own repository if you like.
+/custom/