Enable only on `build`, not on `serve` (configurable)

[tombreit]

Do not slow down docs (re-)generation while documenting and using --serve

• [x] New plugin config option enabled_on_serve
• [x] Respect this new config option
• [x] Document it

class ThisConfig(Config):
    enabled_on_serve = Type(bool, default = False)

class Plugin(BasePlugin[ThisConfig]):
    def on_config(self, config):
        if not self.config.enabled_on_serve and self.is_serve:
            return

[tombreit]

Done in f467fec