docs: replace Sync/Async MkDocs tabs with GitHub-native <details> blocks

[martinkersner]

Closes #166

What

Replace Material-for-MkDocs === "Sync"/"Async" content-tabs with GitHub-native collapsible <details markdown="1"><summary>Sync|Async</summary> blocks in the README and 16 docs pages. Enable md_in_html in mkdocs.yml so fenced code inside <details> still highlights on the ReadTheDocs site.

Why

=== "Sync" renders as literal text on GitHub and the 4-space-indented fenced code beneath it loses syntax highlighting. <details> renders correctly on both GitHub and MkDocs.

Changed

• 34 tab blocks -> <details> across README.md + 16 docs/*.md (docs/index.md is a symlink to README, so it's covered automatically -> 36 blocks total).
• mkdocs.yml: added - md_in_html to markdown_extensions. Left pymdownx.tabbed in place (removal not in scope; harmless once unused).
• Code content preserved verbatim; only the tab wrapper syntax changed (de-indented 4 spaces to sit at column 0 inside <details>).

Verification

• grep: zero === "Sync"/"Async" remain.
• Structural check script: all 36 <details> have markdown="1", a blank line after </summary>, and a blank line before </details> (0 problems).
• mkdocs build --strict: not run (mkdocs not installed locally; per task, no global tooling installed).