bug: template README headings other than h2 render as body text #29232

Summary

On the template Docs tab (/templates/<name>/docs), only <h2> headings in the rendered README are styled. <h1>, <h3> and below render at body size and weight, so a README that starts with # Title shows the title as a plain paragraph, and ### sections are indistinguishable from the text around them.

Steps to reproduce

1. Push a template whose README.md is:

   ---
   display_name: Example
   ---
   
   # Example
   
   Intro paragraph.
   
   ## Section
   
   ### Subsection

2. Open the template's Docs tab.

Expected

# Example is visibly larger than ## Section, which is larger than ### Subsection.

Actual

# Example and ### Subsection look like the intro paragraph. Only ## Section is rendered as a heading.

Cause

Tailwind preflight resets every heading to font-size: inherit; font-weight: inherit. site/src/index.css only restores h2 (font-size: 1.5em; font-weight: bold); h1, h3 and h4 get margins only:

https://github.com/coder/coder/blob/main/site/src/index.css#L769-L787

markdownClassName in site/src/components/Markdown/Markdown.tsx sets heading margins and line-height but no sizes either.

Suggested fix

Give the Markdown component a heading scale (for example [&_h1]:text-2xl [&_h1]:font-bold [&_h3]:text-lg [&_h3]:font-semibold, and so on), or restore sizes for every heading level in index.css next to the existing h2 rule.

Workaround

Only use h2s.

Environment

• Coder v2.37.1+22f4284
• Chrome, Linux