Add corpus release notes page

Mirrors the release log introduced for DeLiKo@DNB: the notes are
maintained as Markdown in docs/corpus-releaselog.{de,en}.md and rendered
by scripts/render-corpus-releaselog.sh (pandoc) into the doc page
templates/custom/doc/corpus/releases.html.ep, which is committed so
deployment never needs pandoc. Run `make corpus-releaselog` after
editing the Markdown.

The render step is explicitly temporary: once Kalamar converts Markdown
doc pages to HTML on the fly, the script, the Makefile target and the
generated .html.ep can be dropped and the Markdown served directly. This
is noted in the script header, the Makefile and the maintenance notes of
both Markdown files.

The page is reachable as "Releases / Ausgaben" under the Corpus menu and
is linked from the corpus page in both languages.

Past releases (2020-I through 2026-I) are reconstructed from the news
page; the DeReKo-2026-II section is a scaffold with TODOs for the
release date, token and text counts, and the actual changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: Id2d52119cf4870c3e7cd5e5afb13e123c0340a88
5 files changed