docs: escape fields

[ras0q]

The description about filename in https://pagescms.org/docs/configuration/#content is broken.

https://github.com/pages-cms/website/blob/796fc0b043206163f739160d97404d6a670c6fd9/src/docs/configuration.md?plain=1#L110

I think it's because {{fields.title}} and {{fields.tags[0].label}} were incorrectly parsed, but since I can't fix it right away, I'll just report it.