Add frontmatter support

Problem/Motivation

In its current form the module renders tags, author, and other metadata as straightforward text and links:

by [Umami](/en/user/6 "View user profile.") 7th March 2025 
 Tags
[Vegan](/en/tags/vegan)
[Vegetarian](/en/tags/vegetarian)
[Oats](/en/tags/oats)
[Breakfast](/en/tags/breakfast)
[Dessert](/en/tags/dessert)

This is fine for most LLM input tasks. However, it the MarkDown is supposed to be fed into a static site generator, then metadata be in frontmatter format.

Steps to reproduce

Request any node via its MD path.

Proposed resolution

As an option, output metadata as frontmatter between triple dashes.
You can use FrontYAML to support the output.

Remaining tasks

User interface changes

API changes

Data model changes