fix(theme-classic): resolve customCss from site dir

[Josh-Cena]

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

Because after #6921, relative paths from plugins are resolved relative to the plugin entry point, specifying relative CSS paths no longer works. (I don't know if it worked before; never tried. It's highly possible that it didn't either.) Anyways, ./src/css/custom.css should definitely work.

Test Plan

Used a relative path in our own config.

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 64	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 83	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: -46 B (0%)

Total Size: 811 kB

Filename	Size	Change
website/build/assets/js/main.********.js	615 kB	-46 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	105 kB
website/build/index.html	38.8 kB

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