fix: ToC generation when importing markdown (Docusaurus v2)

[anatolykopyl]

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

This PR adapts the fix to ToC generation from shuding/nextra#2199 to the toc remark plugin in docusaurus-v2.

Test Plan

Now the title from _pagePartial.md is visible in the ToC of the /tests/pages page.

Also I've added a /tests/pages/partials-tests page for further demonstration.

Test links

Deploy preview: https://deploy-preview-9682--docusaurus-2.netlify.app/

Affected test page: https://deploy-preview-9682--docusaurus-2.netlify.app/tests/pages

New test page: https://deploy-preview-9682--docusaurus-2.netlify.app/tests/pages/partials-tests

Related issues/PRs

#3915

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 88	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟢 92	🟢 100	🟢 100	🟢 100	🟢 90	Report

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[anatolykopyl]

I consider this PR done.
@slorber, could you please check it out? Any tips regarding how this can be improved are appreciated! I'd be glad to make changes to this PR if necessary.

[slorber]

thanks for your work @anatolykopyl

However we don't merge new features and bugfixes to the v2 branch anymore, and will only handle critical security v2 issues until the end of January.

https://docusaurus.io/community/release-process#stable-version

I'll look at your v3 PR when I have time