fix(v2): ignore imports when h1 heading parsing

[lex111]

Motivation

Resolves #4635.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Suppose we have following doc:

---
id: test
---

import BrowserWindow from '@site/src/components/BrowserWindow';

# Title

## Second heading

Results:

Before	After

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

[V1]

Built with commit 2d52db4

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[netlify]

[V2]

Built with commit 2d52db4

https://deploy-preview-4646--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	78
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4646--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 630 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.5 kB	0 B
website/build/assets/js/main.********.js	453 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	27.1 kB	0 B

_{compressed-size-action}

[slorber]

Thanks, not sure it will handle all the edge cases but should be good enough for now.

We should probably ignore exports too and can maybe reuse the regexp from createExcerpt for that.

But supporting multiline exports (like a React comp declared at the top of a MDX file) can be complicated (ex: https://dev.to/sebastienlorber/using-react-hooks-in-mdx-4ei6). Maybe not worth to solve this for now.