Building website is not deterministic

Determinism is a property that allows you to reproduce the exact same website given the exact same files. This would be useful when comparing the effect of changes, since you could run:

```shell
$ zola build --output-dir public-old
$ git switch new-branch
$ zola build --output-dir public-new
$ diff --recursive public-old public-new
```

This currently does not work because random whitespace is inserted by Zola every build, without ever changing the files. To reproduce, run the above command but without switching to a different branch.

The main reason I wanted this is because it could help me review the changes made in #862.

Reproduce the issue with the two `zola build --output-dir` commands from the report, without switching branches, and compare the generated directories recursively. Trace the website build inputs and Zola output to identify where whitespace changes between identical builds; done means repeated builds from the same files produce identical output.

Written by the indexing model from the issue text.