fix(v2): show doc sidebar on pages with case-sensitive paths

[lex111]

Motivation

Since the router configures with case-insensitive paths (by default), we will display the page correctly if the path is written in a different case (creating-Pages -> creating-pages (proper path), and vice versa). Although my personal opinion is that in this case you need to display a 404 error, because this is technically the wrong address, it means that we need to work it out correctly.

However, changing current behavior will result in breaking changes, which is best avoided, so let's leave it as it is. But as noted in issue #2190, in such cases the doc sidebar is not shown. This PR fixes this.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Change or create the id of front matter and filename of any doc page to case-sensitive, eg:

- id: docusaurus-core
+ id: Docusaurus-Core

Make the appropriate changes also in sidebar.js file.

2. Navigate to docs/docusaurus-core (page with wrong path that we will handle) and docs/Docusaurus-Core (new correct path) - the result should be the same.
   Note that, only when going to the wrong page (docs/docusaurus-core) for a second or so will content of 404 page be displayed (which should be)

[docusaurus-bot]

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[bravo-kernel]

@lex111 you might not be a pro but this is pure magic 💃 , I think many users will be happy with this.

Nevertheless I totally agree with your statement and believe that users should fix root causes. For example, I can imagine that the Netlify redirects from mixed case to lowercase could impact SEO (pure intuition). The same for the 404s.

[yangshun]

Looks good, many thanks! Question about the renaming of props, otherwise feel free to self-merge.

Add the "tag: breaking" tag if you decide to go ahead with renaming the prop.