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

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.

After #275 is resolved, start with the spoiler section at severity-static.html#headingSpoiler2 and inspect the existing episode structure. Create the extra episode in the leaner/ folder, move the section there, and update RESOURCES so it is indexed and placed after “how does {cfr} works?”.

Written by the indexing model from the issue text.