Add a Docusaurus cli command to generate documentation without requiring a full build

[CobyPear]

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Hello! Thanks for maintaining the docusaurus-plugin-typedoc. It has suited my needs well. Recently I've wanted to generate the docs through this plugin without running a full docusaurus build, because all I want is the markdown content.

I have opened a draft PR that does this but it needs some work, details here: #355 I am open to continue working on this with some guidance. Thanks!