Drop use of Pandoc, drop support for PDF output

Lars Wirzeniusrequested to merge
liw/drop-pandoc into main

Description

We've wanted to avoid use of Pandoc for parsing Markdown, for a while now. This MR replaces Pandoc as a parser with the pulldown_cmark crate. It also drops Pandoc for generating output. We've decided to only support HTML for output, at least for now.

The HTML output needs a lot of work, but I'd like to get this large change merged so that we have a foundation to work on that's closer to where we want to be. We can then add support for nice things that Pandoc provided us, such as syntax highlighting, table of contents, and more. It'd be nice to not have to do that work ourselves, but it'll also be nice to not have to deal with the unstable AST and command line interface of Pandoc. Furter, it's nice to have fewer runtime dependencies.

Checklist

  • You have read, and followed the guidance in, the CONTRIBUTING guide
  • The MR content has run through a pipeline, or at least has pipelines enabled.
  • There are no licence concerns with this contribution (The README clearly states Subplot is MIT-0)

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