docs: move assets/gh-pages.html to index.html

This matches how we publish the website, keeps things easier locally
if we put it in the same place, even if slightly muddying the top-level
directory semantically (since it's not primarily a website).

It is already excluded from npm publication through the limited
"files" list in package.json.

Author: Krinkle

.github/workflows/coverage.yaml

@@ -40,7 +40,7 @@ jobs:
           cp -R $GITHUB_WORKSPACE/{*.*,assets,coverage,demo,test} .
           # Modifications
           touch .nojekyll
-          cp assets/gh-pages.html index.html
+          cp gh-pages.html index.html
           # Push
           git config user.name "${GITHUB_ACTOR}" && \
           git config user.email "${GITHUB_ACTOR}@users.noreply.github.com" && \