fix: replace HTML flag headers with markdown to prevent emdash rendering

[nikki-ef]

Summary

• Flag names like --verbose on the flag cheatsheet are rendered with an emdash (—) instead of double dashes (--) due to Mintlify's smart typography converting -- in prose contexts
• Replaced <h3><code><a> HTML wrappers with markdown inline code links ([`--flag`](url)) for all 27 flags in docs/reference/flag-cheatsheet.mdx
• Hyperlinks to the command-line reference are preserved
• Mintlify preview: https://bazel-pr-29325.mintlify.app/reference/flag-cheatsheet

Fixes bazel-contrib/bazel-docs#366

Test plan

• Verify on Mintlify preview that flag names render with -- (not —)
• Verify flag names are still clickable links to the command-line reference

🤖 Generated with Claude Code

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[fweikert]

One downside of this change is that we can no longer link to individual flags on this page (since the anchors are being removed). I'm not sure how widely this feature is being used, so for now I'm fine with merging it.

[fweikert]

Ok, I spoke too soon - this change actually breaks the right NavBar on Mintlify. I think we need to keep the h3 tags and just replace <code>/ </code> with triple backticks.

[nikki-ef]

Ok, I spoke too soon - this change actually breaks the right NavBar on Mintlify. I think we need to keep the h3 tags and just replace <code>/ </code> with triple backticks.

Ah I see the issue. I'll push a fix. Thanks for taking a look!