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:

• 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)

• Are you doing the PR on the next branch?

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

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

---

Issue #2135
SO, I manually tested this a bit and it worked as best I could tell.
I ended up having to dig pretty far into code for rendering templates and then bubble information up about whether or not a default template was rendered.
Besides the arguable ugliness of this approach, it also means that it doesn't work for zola check as the build has to happen.
There should be some simpler way to check all of the templates without building...I will think about it now that I'm getting more familiar with the code and I am also open to suggestions. I may be able to work on this later today yet, we'll see.

IF this approach is acceptable, at the very least I want to do a cleanup pass and add some comments/documentation.

[Raymi306]

I've begun rewriting this.
My new plan is to add a method to Site, "get_default_templates".
This method will iterate through all pages and sites of the library.
I will write some functions to determine if a page or site is using a default template, collect the results, and return a vec either of pathbufs or strings

[Raymi306]

See #2205