docs: open ODT output files with File.Create instead of File.OpenWrite

[vaceslav]

Follow-up to #138 (closed), from the independent review of the OpenDocument support.

Problem

docs/for-developers/opendocument.md recommended File.OpenWrite for the output stream. File.OpenWrite does not truncate. When it overwrites an existing, longer file, the old bytes after the new content remain, and the result is a corrupt ZIP. #221 now cuts off seekable outputs in OdtPackage.Save, but the docs should not depend on that or teach the pattern.

Fix

• The "Streams and Byte Arrays" sample uses File.Create, which creates or truncates the file.
• The requirements list says the output stream only has to be writable. It recommends File.Create for files and explains the truncation problem with File.OpenWrite.
• The "Memory Use" note on cutting off seekable outputs now says to prefer File.Create anyway.
• I checked every doc, README and XML doc for File.OpenWrite. The DOCX docs (quick-start.md, Examples.md) already use File.Create and mention File.OpenWrite only as the write-only stream that fails, so they stay as they are. The OdtPackage XML doc is part of the package work and is not changed here.

Tests

• OpenDocumentSamplesTests mirrors the changed sample, which now uses File.Create (renamed to OdtTemplateProcessor_BytesAndOutputStream). OdtPackageTests still covers write-only outputs and truncation.
• mkdocs build --strict, the Release CI build, all tests on net10/9/8, dotnet format --verify-no-changes and dotnet pack pass.

Public API impact: none (documentation only).

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