Support for loading custom Markdown files

[danieleds]

This PR adds the following features:

• support for loading arbitrary markdown files from the file system tree
• support for _spec_.{yaml,yml,json} files, which do not generate a new category
• support for sorting categories and items in the sidebar using _category_ or front matter in custom mdx files

Solves:

• https://github.com/cloud-annotations/docusaurus-openapi/issues/135
• https://github.com/cloud-annotations/docusaurus-openapi/issues/136
• https://github.com/cloud-annotations/docusaurus-openapi/issues/137 (using _spec_, but we can change this)

Example

File tree (notice examples/custom-overview.md, examples/petstore/_spec_.yaml, examples/petstore/my-intro.md):

Results:

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@bourdakos1 I would love your review on this.

There are a lot of changes in the diff, but most of the stuff (the one in packages/docusaurus-plugin-openapi/src/docs/) has been taken and "tree-shaked" from the original plugin-content-docs plugin (they don't export those functions, unfortunately).

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[bourdakos1]

Is this ready for review?

[danieleds]

@bourdakos1 It is ready, I am just making some minor linting fixes

[bourdakos1]

It will probably take me a few days to review this, but I’m excited for the update 😁

[danieleds]

fixed a bug where the slug property wasn't properly recognized

[bourdakos1]

The "My Custom Petstore" / "My Intro" page doesn't load for me

[danieleds]

My bad, I forgot to push stuff. Could you try it again?

[joanjane]

Great feature! Do you have plans to release a new version of the package on npm? I'm interested using this feature too :)

Thank you!