Fix formatting parser issues in headings, units, and media docs

[dilipom13]

Summary

• Fixes formatting parser issues when Markdown is published to make.wordpress.org/docs
• Moves code fences outside notice shortcodes ([tip], [warning], etc.)
• Replaces HTML tags in backticks with <code> elements where the parser strips them
• Fixes non-breaking space rendering in units-of-measurement examples

Files changed

Issue #26:

• headings.md, units-of-measurement.md, media.md

Issue #12 (remaining):

• procedures.md, tables.md, heading-targets.md, image-links.md, code-examples.md, placeholders.md

Fixes #26
Addresses #12