docs: define practices for third-party extensible APIs

[samuelmaddock]

A few of Electron's existing APIs don't do a good job of supporting extensibility for third-party libraries. Let's try to define best practices to guide future API designs with third-party extensibility in mind.

protocol.registerSchemesAsPrivileged

• Replaces all schemes without an option of appending
• Known issue for third-party libraries [Feature Request]: registerSchemesAsPrivileged cannot be used by libraries  electron#40362
• Requires monkey patching to workaround (see Sentry's example)

session.setPreloads / session.getPreloads

• Requires replacing all preloads rather than adding a single preload

session.setPreloads([
  ...session.getPreloads(),
  path.join(__dirname, 'new-preload-script.js')
])