feat: add markdown output plugin to copy markdown files alongside HTML

[clavin]

Description of Change

Adds a custom Docusaurus plugin to emit Markdown files alongside the generated HTML files.

When Docusaurus compiles a markdown route, it creates an index.html file within that route. This plugin copies the source file to a index.md file alongside it.

Example

Source tree

─ docs/
  ├─ flowers.md
  └─  ...

Output tree

─ docs/
  ├─ flowers/
  │  ├─ index.html
  │  └─ index.md     # New!
  └─ ...

Checklist

• This PR was not created with AI. (PRs created mainly with AI will be closed. They waste our team's time. We ban repeat offenders.)

[clavin]

I plan to pair this with a change to send some clients these markdown files: https://github.com/electron/infra/pull/161

[clavin]

@erickzhao could you also merge this for me? 🙏

[dsanders11]

Seems reasonable, and when running yarn build I can see the expected output, but I was not able to view the index.md files with the dev server spun up by yarn start. I'm guessing that's just something that's not wired up in their dev server (since it doesn't exist), so happy to merge this and follow up if needed.

[dsanders11]

Looks like this may not be being served as UTF-8? This page for example (https://www.electronjs.org/docs/latest/why-electron/index.md) is rendering some characters incorrectly.

[dsanders11]

This probably doesn't play nice with i18n as those are prefixed by the language code. I think ideally we'd also serve the translated Markdown, so this should be updated to take that into account.

[clavin]

🤔 Weird, but makes sense that there's encoding issues, browsers be like that. In the infra PR I've specified the encoding in the Content-Type header--will try it out locally to make sure that fixes it. (Though direct links like yours will still have that issue.) Hopefully, non-browser clients to be more likely to default to UTF-8 or detect it.

[clavin]

Maybe there's a way to set the content encoding when we sync with azcopy 🤔