move spoiler on "why we use `density()`?" in a new extra episode after `how does {cfr} works?`

[avallecam]

after solving #275 , we could move section

• https://epiverse-trace.github.io/tutorials-middle/severity-static.html#headingSpoiler2

to a new extra episode also indexed in RESOURCES in website, located at the leaner/ folder.

This is because understanding density depends on the inner work too. And it would reduce methods content in episode, and keep it as optional to review later.