https://github.com/HenryQW/mercury_fulltext

The directions there are pretty clear. You've gotta set up the mercury parser API service (I used docker) and then enable the plugin for the feeds you want to apply it to.

Alternatively you could use the Readability plugin that ships with tt-rss, but I have no idea how effective it is as I never tried it.

Finally, you could stand up the RSS full text proxy:

https://github.com/Kombustor/rss-fulltext-proxy

That service standa between your RSS feed reader of choice and the RSS feed supplier and does the scraping and embedding.