Add [[nodiscard]] to view-returning public methods

[DaviBaum]

Summary

• Annotate public methods returning non-owning view types with [[nodiscard]] across both v1/ and v_noabi/ namespaces in bsoncxx and mongocxx headers.
• Target return types include document::view, array::view, element::view, string_view, types::view, bson_value::view, view_or_value, and optional<> wrappers around these types.
• Silently discarding a view return value is almost always a bug (dead code or a missing check). This change surfaces those issues at compile time via -Wunused-result.

Scope

• 140 header files modified, ~434 annotations added.
• No runtime behavior changes — [[nodiscard]] is a compile-time warning only.
• No ABI impact.
• Conversion operators (operator view(), operator bool()) and XMACRO-generated get_*() methods are intentionally excluded.
• Formatted with uv run --frozen etc/format.py.

Test plan

• cmake --build build --target bsoncxx_shared mongocxx_shared passes with zero errors.
• CI should confirm no new warnings or test failures.

[kevinAlbs]

Thank you for the contribution @DaviBaum. Closing since we do not want to proceed with this change. [[nodiscard]] was added in C++17 but the C++ driver API supports C++11. Even for C++17, I expect this could break compile for existing consumers and would require a major version release.