Mark Planned Content with a dagger already in the section title

[nomeata]

While the manual is already in use, but incomplete, it is somewhat disappointing to hopefully click on a section title, like “Quotients”, to only then see “Planned Content”. We can maybe avoid this anti-dopamine kick for our users if all sections with Planned Content are marked as such in the TOC, for example by writing Quotiens†, in a clever pun for inaccessible names in Lean.

[david-christiansen]

Not a bad idea - but I hope that there will be no such sections quite soon, so I'm not inclined to spend a lot of time on this part.