[TASK] Revise "Ajax in the backend" chapter

[brotkrueml]

• Move PHP/JS snippets into separate files
• Recommend to add the extension name to the route key for better uniqueness
• Substitute included BackendAjaxRoutes.rst.txt with a custom one to be in-line with the PHP controller example namespace (the BackendAjaxRoutes.rst.txt is still in use in another place)
• Remove the ResponseInterface constructor argument list: the response factory is used, so no need to list constructor arguments
• Use ES6 instead of RequireJS in JavaScript example

This is a preparation for merging the "Ajax" chapter with the "Ajax, clientside" chapter.

Related: #2331
Releases: main, 12.4

[github-actions]

The backport to 12.4 failed:

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

stderr

error: could not apply 00fb1d21... [TASK] Revise "Ajax in the backend" chapter
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".

stdout

Auto-merging Documentation/ApiOverview/Backend/Ajax.rst
CONFLICT (content): Merge conflict in Documentation/ApiOverview/Backend/Ajax.rst

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-12.4 12.4
# Navigate to the new working tree
cd .worktrees/backport-12.4
# Create a new branch
git switch --create backport-3471-to-12.4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 00fb1d21023a9af41f155f38ea058eb77ed8713c
# Push it to GitHub
git push --set-upstream origin backport-3471-to-12.4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-12.4

Then, create a pull request where the base branch is 12.4 and the compare/head branch is backport-3471-to-12.4.