fix(v2): make config validation less strict

[teikjun]

Motivation

This PR modifies the config validation for the following fields:

1. navbar.title
2. rehypePlugins
3. routeBasePath
4. scripts
5. stylesheets

navbar.title

It allows the following values for navbar.title: '', null, undefined. This fixes #3117.

rehypePlugins

The schema for rehypePlugins has been modified. It now accepts an array, whose items are either:

• a function (because the rehypePlugins are of function type).
• an array of [function, object] in Babel style (See https://v2.docusaurus.io/docs/markdown-features/#configuring-plugin-options for an example)

This fixes #3125.

routeBasePath

It accepts the empty string '' now. This is useful for removing /docs from url while having a landing page. This fixes #3126.

scripts and stylesheets

scripts accepts both defer and async now.
stylesheets accepts unknown fields now.
This fixes #3128

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

navbar.title

1. In /website/docusaurus.config.js, change navbar.title to '' or null, or remove the field.
2. There should be no validation errors in all cases.

rehypePlugins

1. Follow the steps to reproduce Error when including rehype plugins #3125
2. There should be no validation errors thrown now
   (I didn't add an automated test for rehype plugin because the dependency will only be used for testing)

routeBasePath

1. Follow the steps to reproduce Can no longer remove /docs from the URL since v59 #3126. For step 3, set routeBasePath: ''
2. There should be no validation error raised

scripts and stylesheets

1. Refer to the steps to reproduce Validation too strict for stylesheet and scripts  #3128. Add the config for scripts and stylesheets to D2 website.
2. There should be no validation errors raised.

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[slorber]

Thanks @teikjun