Add new ol-mapserver.js version, update docs with demo links

[geographika]

Add new OpenLayers MapServer JS lib for MapServer/MapServer#7290

Additional docs updates:

• Add note about writeable folders
• Add links to https://demo.mapserver.org
• Formatting

[github-actions]

The backport to branch-8-4 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply f5c1f04b... Add new ol-mapserver.js version, update docs with demo links
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging en/cgi/openlayers.txt
CONFLICT (content): Merge conflict in en/cgi/openlayers.txt

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-branch-8-4 branch-8-4
# Navigate to the new working tree
cd .worktrees/backport-branch-8-4
# Create a new branch
git switch --create backport-1022-to-branch-8-4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick f5c1f04b3092559efcf98fcf36e64125d90744f5,53e36434724da83d0a281e0f2f8946eec7572b14
# Push it to GitHub
git push --set-upstream origin backport-1022-to-branch-8-4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-branch-8-4

Then, create a pull request where the base branch is branch-8-4 and the compare/head branch is backport-1022-to-branch-8-4.