docs: added reference for JS Client

[shahednasser]

This PR adds a new reference to the documentation covering the Medusa JS Client.

• Added new Typedoc plugins to customize the generation of the reference
• Created a custom Typedoc plugin to add frontmatter data at the top of specific markdown files. This is necessary to display the sidebar in the reference on some of the deeply-nested pages that do not appear on the sidebar.
• Added a new link in the sidebar "JS Client Reference"
• In the JS Client Reference, added an overview page to introduce the client and how the reference can be used.
• Added a new sidebar for the JS Client reference. Due to the hierarchy of the reference package and to make the reference easier to use, the sidebar is manually created and not autogenerated.
• Added in the README of the JS Client a link to the new reference and changed the homepage URL of the package.

I did not create a GitHub action for this reference as it might mess up the sidebar. It's better to attend to any changes manually to make sure the documentation always functions properly.

Closes #1570

Question: Should we have in the navigation bar a "References" dropdown or should we keep the references linked separately?

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Latest commit: 2c1fd39

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[srindom]

LGTM! Let's get this in!

Regarding your question I think it would be cool with a drop-down :)