docs: add tip on browser extensions to perf guide

[patak-dev]

Description

See https://twitter.com/patak_dev/status/1745427465617433005

Full page reload of 10k modules with dev tools opened:

4.2s - With Tampermonkey + uBlock
3.3s - Only uBlock
2.5s - No extensions
2.0s - Incognito

Without dev tools opened, Vite's dev server loads the 10k modules in 1.7 seconds on my M1 (hitting all 304). Without extensions, the difference when having dev tools opened is only ~10%

2.9s - Tampermonkey + uBlock
2.3s - Only uBlock
2.2s - No extensions
1.7s - Incognito

Add the new section at the top because it is short and it should help that more people will see it.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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