fix(utils): strip MDX export blocks before heading detection in parseMarkdownContentTitle

[okxint]

MDX export blocks appearing before the first heading were not stripped before parseMarkdownContentTitle ran its heading regex, causing the function to match content inside the export block instead of the actual document title. Strip export blocks first so heading detection works correctly regardless of MDX preamble.

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[okxint]

Closing as I'm unable to complete the CLA signing required to proceed. Thanks for the review!