Add ability to specify sub categories in sidebar.json

[WillBrock]

Motivation

Some users would like to be able to specify sub categories in the sidebar as discussed in #410.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Added unit tests for the functions that were modified and visually verified existing sidebars function the same as well as the new sub categories.

Example sidebars:

{
  "docs": {
    "First Category": {
      "Sub Cat One" : ["doc2", "doc3"],
      "Sub Cat Two" : ["doc4", "doc5"]
    },
    "Second Category" : ["doc8"]
  }
}

{
  "docs": {
    "First Category": {
     // Empty key will display no header
      "" : ["doc1"],
      "Sub Cat One" : ["doc2", "doc3"],
      "Sub Cat Two" : ["doc4", "doc5"]
    },
    "Second Category" : ["doc8"]
  }
}

First attempt at implementing this, let me know if some things should be changed. I changed a couple of the functions around to make them easier to write tests against. In the future we could add tests to the rest of the functions in readMetadata.js as well.

Note: We could maybe do some type of accordion for the sub categories if wanted.

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[yangshun]

@WillBrock Sorry I clicked the wrong button. Could you resubmit the PR? Very sorry.

[WillBrock]

@yangshun no problem. Added it in #892