fix: make headers on the website bold

[joyeecheung]

Description

This makes the posts more readable.

Validation

I didn't build the website locally but here is what I see in the browser when I applied changes to base.css.

Before (difficult to tell changes apart):

After (easier):

Related Issues

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run npx turbo lint to ensure the code follows the style guide. And run npx turbo lint:fix to fix the style errors if necessary.
• I have run npx turbo format to ensure the code follows the style guide.
• I have run npx turbo test to check if all tests are passing.
• I've covered new added functionality with unit tests if necessary.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
nodejs-org	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 29, 2023 2:43pm

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO	Report
/en	🟢 94	🟢 97	🟢 92	🟢 92	🔗
/en/about	🟢 100	🟢 95	🟢 92	🟢 92	🔗
/en/about/previous-releases	🟢 98	🟢 93	🟢 92	🟢 92	🔗
/en/download	🟢 100	🟢 95	🟢 92	🟢 92	🔗
/en/blog	🟢 98	🟢 95	🟢 92	🟢 92	🔗

[github-actions]

Unit Test Coverage Report

Lines	Statements	Branches	Functions
	90.37% (366/405)	77.31% (92/119)	89.18% (66/74)

Unit Test Report

Tests	Skipped	Failures	Errors	Time
71	0 💤	0 ❌	0 🔥	5.693s ⏱️

[ovflowd]

cc @nodejs/nodejs-website I'm fast-tracking this as a very simple change