Added CODE_OF_CONDUCT.md and CONTRIBUTING.md files - #992

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-dotnet@v4, github/codeql-action/analyze@v3, github/codeql-action/init@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Possible null reference argument for parameter 'instance' in 'object? MiniExcelMemberAccessor.GetValue(object instance)'.

Possible null reference argument for parameter 'key' in 'object IDictionary<string, object>.this[string key]'.

Argument of type 'List<MiniExcelColumnMapping>' cannot be used for parameter 'mappings' of type 'List<MiniExcelColumnMapping?>' in 'CellWriteInfo[] AsyncEnumerableWriteAdapter<T>.GetRowValues(T currentValue, List<MiniExcelColumnMapping?> mappings)' due to differences in the nullability of reference types.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Possible null reference argument for parameter 'name' in 'int IDataRecord.GetOrdinal(string name)'.

Possible null reference argument for parameter 'key' in 'object IDictionary<string, object>.this[string key]'.

Argument of type 'List<MiniExcelColumnMapping>' cannot be used for parameter 'mappings' of type 'List<MiniExcelColumnMapping?>' in 'CellWriteInfo[] EnumerableWriteAdapter.GetRowValues(object currentValue, List<MiniExcelColumnMapping?> mappings)' due to differences in the nullability of reference types.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Cannot build an overlay database because build-mode is set to "undefined" instead of "none". Falling back to creating a normal full database instead.