Description of Change

This is the result of the fix Windows codesigning project I signed up for at the last maintainer summit. A quick summary of all the changes:

• @electron/windows-sign has been created and is now integrated in @electron/packager, electron-winstaller, and electron-wix-msi.
• All our tooling is now compatible with EV code signing certificate workflows that involve "cloud signing solutions", since you can call a custom binary or JavaScript hook for each file that needs to be signed - even inside Squirrel.Windows, if required.
• Analogous to @electron/osx-sign, all tools now use the same configuration (windowsSign), so wrapping your head around things is a lot easier.

This PR updates our documentation to document those changes.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation, tutorials, templates and examples are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: no-notes