I come to realise, that quite a lot of the grav features do not apply to the RTFM Learn2 framework.
I found this:
https://getgrav.org/forum#!/getgrav/plugin-development:adding-searchable-tags-on-r
It says there here, It'd be difficult to realise, for this feature apparently is designed for blog environments.
However, with some tweaking it could be done. Has it been done?
I can't find anything else on it.