Hi,

The core guidelines recommend catching by const& for all but very small value types. One of my colleagues filed a PR internally with catch (nlohmann::json::type_error& e), instead of catch (const nlohmann::json::type_error& e), with the explanation that it was copied from the documentation. The error here holds a string however, so it is not a simple type that fits in a single register, I think const & is appropriate. This PR updates the documentation's exception catching to be all const.

Automated replace performed with the following command:

find ./ -type f -name *.cpp -print0 | xargs -0 sed -i -e 's|catch (json|catch (const json|g'

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
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