Create a section for all the intermissions from sakubi

[Morgawr]

Sakubi shows intermissions between lessons. I don't want to break down the mdbook chapter order by adding intermissions, so I'm thinking of having a special appendix at the end of the book with all optional content (optional lessons, intermissions, hidden lessons, etc) and directly link to them as extra content in the main guide.

I just need to start uploading them as I extract them from my draft.

Also, some of the hidden lessons will probably become main lessons, and some will go into a spillover (翌々日?) extra grammar guide later.