fix(v2): fix multi-instance mdx loaders not sandboxed correctly

[slorber]

Motivation

When 2 docs plugin instance use similar paths like /community and /community-2, a doc like /community-2/myDoc.mdx is actually catched by both mdx loaders at the same time.

The solution is to add a trailing slash in mdx loader include path so that the /community/ will not catch /community-2/myDoc.mdx

Fixes #3801
Fixes #3818

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

not sure how to test that

Related PRs

Superseed #3969

[netlify]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	79
🟢 Accessibility	99
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-3970--docusaurus-2.netlify.app/classic/

[github-actions]

Size Change: 0 B

Total Size: 155 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/blog/2017/12/14/introducing-docusaurus/index.html	20.7 kB	0 B
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	5.82 kB	0 B
website/build/main.********.js	111 kB	0 B
website/build/styles.********.css	17.6 kB	0 B

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