Fix: Reduce multiple H1s to H2s in Distributed Agent Runtime page (#6412)

## 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](https://github.com/microsoft/autogen/issues/6090)

## Checks

- [x] 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.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>