docs: async guidance for web applications (#17)

[vaceslav]

Follow-up to #17 (closed as not planned): instead of an async API, document how to do the I/O around the synchronous API asynchronously.

Changes

• FAQ: new "Is there an async API?" entry (why not, input/output handling in ASP.NET Core); the web-apps entry links to it.
• Developer quick start: "Web Applications (Async I/O)" section with async buffering of uploads.
• Examples.md: "Uploaded Templates (Async I/O)" controller example using TemplateProcessor (.docx and .odt), DetectFormat for the content type, and a note why Response.Body cannot be the output.
• Tests: DeveloperGuideSamplesTests mirrors the new samples with a non-seekable upload stream that throws on synchronous reads (as Kestrel with AllowSynchronousIO = false).

Checks

• Documentation tests: 68/68 passed on net10.0, net9.0 and net8.0
• dotnet build -c Release -p:ContinuousIntegrationBuild=true: 0 warnings, 0 errors
• dotnet format --verify-no-changes: clean
• mkdocs build --strict: OK
• LibreOffice round trips: not run (no library or OpenDocument code changed)

Public API impact

None.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!