nxml-mode: Add schemas for Mono/.NET related files

nxml-mode: Add schemas for Mono/.NET related files

* etc/schema/dotnet-appconfig.rnc:
* etc/schema/dotnet-packages-config.rnc:
* etc/schema/dotnet-packages-props.rnc:
* etc/schema/dotnet-resx.rnc:
* etc/schema/msbuild.rnc:
* etc/schema/nuget.rnc:
* etc/schema/nuspec.rnc: New files.
* etc/schema/README: Document copyright status of above new files.
* etc/schema/schemas.xml: Use above new files to support Mono/.NET
development related XML files.

This change was discussed in:
https://lists.gnu.org/r/emacs-devel/2024-02/msg00638.html

(cherry picked from commit e74eedaa2e34d7fd1c3ba9bd2632468bf36d5774)