'Added Jekyll example with explanation' (#4)

Reviewed-on: https://codeberg.org/Codeberg-CI/examples/pulls/4

Jekyll/_config.yml

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+title: My Jekyll Site
+url: https://www.myjekyllsite.TLD
+description: This is a beautiful page.
+remote_theme: pages-themes/cayman@v0.2.0
+plugins:
+- jekyll-remote-theme # add this line to the plugins list if you already have one
+# The keep_files are needed to preserve git metadata when building
+keep_files: [ .domains, LICENSE, .git, .gitignore ]
+exclude: [ domains ]

Jekyll/cpages.yml

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+# cpages.yml
+#
+# Takes a repository with jekyll source, generates the static site and
+# pushes the result to codeberg pages
+#
+# Needs a codeberg access token (cbtoken) as secret in woodpecker config
+# Also uses another secret (cbmail) with email address for git config
+#
+# CBIN must be replaced with the source repo
+# CBOUT must be replaced with the target codeberg pages repo
+# CBUSER must be replaced with the user/org
+#
+# See the _config.yml file for the important keep_files: line to preserve
+# git metadata during build
+#
+# We also assume a domains file in the source repo that gets copied to
+# .domains in the target repo so codeberg pages works for custom domains
+#
+pipeline:
+  build:
+    # Use the official jekyll build container
+    image: jekyll/jekyll
+    secrets: [ cbtoken, cbmail ]
+    commands:
+      # Avoid permission denied errors
+      - chmod -R a+w .
+      # Set up git in a working way
+      - git config --global --add safe.directory /woodpecker/src/codeberg.org/CBUSER/CBIN/_site
+      - git config --global user.email "$CBMAIL"
+      - git config --global user.name "CI Builder"
+      - git config --global init.defaultBranch pages
+      # clone and move the taret repo
+      - git clone -b pages https://codeberg.org/CBUSER/CBOUT.git
+      - mv CBOUT _site
+      - chmod -R a+w _site
+      - cd _site
+      # Prepare for push
+      - git remote set-url origin https://$CBTOKEN@codeberg.org/CBUSER/CBOUT.git
+      - cd ..
+      # Run Jekyll build stage
+      - bundle install
+      - bundle exec jekyll build
+      # Only needed for custom domains
+      - cp domains _site/.domains
+      # Push to target
+      - cd _site
+      - git add --all
+      - git commit -m "Woodpecker CI Jekyll Build"
+      - git push