Add GitHub Actions workflow

.github/workflows/continuous-integration.yml

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+name: osu-wiki continuous integration
+
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  ci:
+    name: remarkjs, changed files
+    runs-on: ubuntu-latest
+    steps:
+      - name: sparse checkout
+        shell: bash
+        run: |
+          # repository url, utilising provided github credentials
+          REPOSITORY="https://${{ github.actor }}:${{ github.token }}@github.com/${{ github.repository }}.git"
+          # merge commit ref name (with refs/heads/ stripped out)
+          BRANCH="${GITHUB_REF/#refs\/heads\//}"
+
+          git version
+          # clone without blobs; don't checkout to avoid fetching them anyway
+          git clone --filter=blob:none --no-checkout ${REPOSITORY} .
+          git config --local gc.auto 0
+
+          # set up sparse checkout
+          git sparse-checkout init
+          git sparse-checkout set '**/*.md' '**/.remark*' '**/*.json'
+
+          # fetch the merge commit ref
+          git -c protocol.version=2 fetch --no-tags --prune --progress --depth=2 origin +${GITHUB_SHA}:refs/remotes/origin/${BRANCH}
+          git checkout --progress --force -B $BRANCH refs/remotes/origin/$BRANCH
+
+      - name: setup node
+        uses: actions/setup-node@v1
+
+      - name: audit dependencies
+        run: npm audit
+
+      - name: install remark
+        run: npm install
+
+      - name: run remark on changed files
+        # stdout is discarded (remark prints files being checked there)
+        run: git diff ${{ github.sha }}^ ${{ github.sha }} --name-only '*.md' | xargs npx remark -qf --no-stdout --report=vfile-reporter-position --color

.gitignore

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+# prevent accidental addition of files from local remark runs
+node_modules/