Support custom corpus documentation templates

Mount a "custom" directory into the kalamar container, so users can
describe their own corpus in the KorAP user interface: Kalamar serves a
documentation page from there whenever it does not ship one itself, so a
doc/corpus.html.ep becomes the corpus page. Ship examples for that page,
for an optional references page, and for the corpus part of the
references page.

The directory defaults to ./custom and can be relocated with
KALAMAR_TEMPLATES.

Also allow overriding the kalamar image via KALAMAR_IMAGE, which is
needed to try this out on a Kalamar built from an unreleased version.

The references page needs the citation partial of Kalamar 0.66 or later.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: I8d964c996c15aa1d4ea85f18be223d737ed6d528
diff --git a/custom/.gitignore b/custom/.gitignore
new file mode 100644
index 0000000..01e9755
--- /dev/null
+++ b/custom/.gitignore
@@ -0,0 +1,5 @@
+# Only the examples are part of the repository; the templates you derive from
+# them are yours and stay untracked.
+*.html.ep
+*.html.md
+!*.example