This is a follow-up to the following PR.

• Add a new setting to add custom C# defines in the C# project 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>$(DefineConstants);FOO</DefineConstants> and not <DefineConstants>FOO</DefineConstants> to avoid writing over the Godot constants.
• Add a link to the official documentation (not sure which page would be best, maybe this one).

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