Clean up docs on native Node modules

[jryans]

#11125 adds integration and docs for manually enabling Seshat for desktop Riot.

At the moment it recommends:

• yarn add matrix-seshat in electron_app for initial install
• Usage of electron-build-env for rebuilds

There are a few issues here that leave some room for improvement:

• The main purpose of electron-build-env seems to be ensuring that certain env vars are set, including npm_config_runtime which neon seems to need for at least Windows
  • This suggests we need some mechanism of settings these env vars for both initial install and rebuilds
• Installing matrix-seshat as a dependency will compile a build for plain Node as part of it's own scripts, but we probably need the adjusted Electron build instead, which may awkwardly mean building it twice with electron-build-env from the parent Electron app as well
  • Despite the wasted time of two builds, this seems to be what the Neon example for Electron suggests

[jryans]

We now have an auto-build process after #12452, so I suppose all that's left here is to clean up docs on our support for native node modules.

[t3chguy]

Fixed