Support monorepos with multiple docs plugins

I am creating this issue to track all the issues related to using multiple docs plugins without a single repo.

Below is an example of my setup:

```
- . <- plugin-content-docs path
- .git
- tmp/logs
- .tmp/more-logs
- readme.md
- package.json
- docs <- docs for the monorepo
- sidebar.js
- readme.md
- packages
- docs-app
- package.json
- docusaurus.config.js
- pkg1 <- plugin-content-docs path
- docs
- sidebar.js
- readme.md
- pkg2 <- plugin-content-docs path
- docs
```

There are 3 docs plugins in use here. One for the monorepo root package, and two others for other packages.

I have indicated where the plugin's `path` config is set.

For the monorepo root docs plugin I set the `path` to be the root rather than the `docs` folder. This allows me to include the `readme.md` file in my docs.

# Issues

- [x] [Allow custom chokidar watcher configuration](https://github.com/facebook/docusaurus/issues/4055)
- [ ] [Allow custom excludes for each content-docs-plugin's webpack mdx loader rule](https://github.com/facebook/docusaurus/issues/4035#issuecomment-764899925)
- [ ] [Allow referencing markdown links from the repository root](https://github.com/facebook/docusaurus/issues/4039)