Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

If you have multi-instances and versioning and you're trying to import partial MDX to a file from cut version, you'll get console error. This happens because a paths to the directory with the next version docs and directory with docs for some version starts with the same, i.e "**/instance" and "**/instance_versioned_docs/**". So, isMDXPartial() uses wrong directory as a root directory as a root. We need to append slashes to contentDirs, so that "**/instance/**"(slash added) will not mix up with "**/instance_versioned_docs/**"

Example:

In my docs folder I have a folder "_includes" with MDX partials. I use multi-instances, "instance" is a name of one of the instances. I import partials like that:
import Partial from '@site/instance/_includes/_partial.mdx';
Then I cut a new version and want to update the imports to "versioned_docs" path, so I use:
import Partial from '@site/instance_versioned_docs/version-1.0.0/_includes/_partial.mdx';

That's the error I get:
[ERROR] MDX loader can't read MDX metadata file "path-to-my-repo\.docusaurus\docusaurus-plugin-content-docs\instance\site-instance-versioned-docs-version-1-0-0-includes-partial-mdx-d57.json". Maybe the isMDXPartial option function was not provided?
[ERROR] Error: ENOENT: no such file or directory, open 'path-to-my-repo\.docusaurus\docusaurus-plugin-content-docs\instance\site-instance-versioned-docs-version-1-0-0-includes-partial-mdx-d57.json'
[INFO] Docusaurus version: 2.2.0
Node version: v16.14.0
error Command failed with exit code 1.

The same thing when I use relative paths.