Document how to treat source generator diagnostics as errors

[yvanlo]

Closes #156

Adds a "Treat diagnostics as errors" section under Diagnostics, with:

• the list of warning IDs (MSG0001–MSG0010)
• a WarningsAsErrors example
• an .editorconfig example

Also links to it from the Quick Start (4.1).

Both approaches were tested locally: a message without a handler (MSG0005) fails the build.