Fix: Reduce multiple H1s to H2s in Distributed Agent Runtime page

[LuluZhuu]

Why are these changes needed?

This PR fixes the issue of multiple <h1> headers in the Distributed Agent Runtime documentation page. The page has more than one <h1> which violates semantic HTML structure. This fix downgrades the inner section headings (e.g., "Cross-Language Runtimes", "Next Steps") to <h2>.

Related issue number

Related issue: #6090

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.29%. Comparing base (0e9744c) to head (8e152fe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6412      +/-   ##
==========================================
- Coverage   78.30%   78.29%   -0.01%     
==========================================
  Files         221      221              
  Lines       16184    16184              
==========================================
- Hits        12673    12672       -1     
- Misses       3511     3512       +1     

Flag	Coverage Δ
unittests	78.29% <ø> (-0.01%)	⬇️

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