frontmatter: Move frontmatter content into vm.frontmatter

[petrjoachim]

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Switching frontmatter config from vm.config to separate vm.frontmatter solves cross-polluting when going from page with frontmatter to page without it.

Also switching from attributes.config to plain attributes as the first one requires every page to have frontmatter in format below, which is unnecessary and counterintuitive (beacuse of missing documentation ofc).

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config:
  - author: @petr
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Closes #548