Document the management of `.csproj` files in .NET projects

This is a follow-up to the following PR.
- godotengine/godot#94939

As discussed, it would be nice to write a documentation page about what Godot does (and doesn't do) when it comes to `.csproj` files. I personally think the following points should be included.
- Explicitly state that users can manage the content of their `.csproj` file as they see fit.
- Explain that Godot will update the `Sdk` version if it needs to, but that should be the only thing it ever overwrites.
- Explain the content of the default `.csproj` file, and what users should not touch to have their project working properly in Godot.
- Explain how to define constants, since it is a very common use case. Maybe even note that when editing as text, one should do `$(DefineConstants);FOO` and not `FOO` to avoid writing over the Godot constants.
- Add a link to the official documentation (not sure which page would be best, maybe [this one](https://learn.microsoft.com/en-us/visualstudio/msbuild/msbuild?view=vs-2022#project-file)).

I think this should probably be done in a brand-new page under the C# manual section.