Support YAML front matters

YAML front matters (as used by [assemble](https://assemble.io/docs/YAML-front-matter.html), [hugo](https://gohugo.io/content-management/front-matter/) [static website renderers], [zettlr](https://docs.zettlr.com/en/core/yaml-frontmatter/), [obsidian](https://help.obsidian.md/Advanced+topics/YAML+front+matter) [editors/note-taking tools], and [pandoc](https://pandoc.org/MANUAL.html#extension-yaml_metadata_block)) should either be ignored, or treated as metadata (rendered possibly as a table, or possibly in the box with the Contents).

rendering it as a header also has the side effect that, because there is no `` tag & the first header becomes the title, that the raw YAML (with line breaks stripped) becomes the text in the browser tab.

an example front matter & first header from a file that is notes from reading a book... in this case the ideal title would be `Supernatural, Michael Heiser (2015)` but it's actually `citekey: heiserSupernaturalWhatBible2015 ebook: - https://read.amazon.com/?_encoding=UTF8&asin=B016LT2YHA`

``` markdown
---
citekey: heiserSupernaturalWhatBible2015
ebook:
- https://read.amazon.com/?_encoding=UTF8&asin=B016LT2YHA
---

# *Supernatural*, Michael Heiser (2015)
```

The issue names no files, tests, or entry points. Start by reproducing the supplied Markdown example in the viewer and trace how its YAML block is parsed and how the first header determines the browser title. Done should mean front matter no longer becomes rendered header text or the tab title, with the intended metadata behavior settled.

Written by the indexing model from the issue text.