Add default template warning

[Raymi306]

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

The place to discuss new features is the forum: https://zola.discourse.group/ If you want to add a new feature, please open a thread there first in the feature requests section.

Sanity check:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• [x] Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• [x] Have you created/updated the relevant documentation page(s)?

---

Rewrite of #2200 This should be less ugly with way fewer lock acquisitions, and it actually works for zola check. I have only done some simple manual tests so far.