[WIP] Start transforming documentation toggles into pure css ones

[GuillaumeGomez]

This is a first draft of #83332. I precise: this is a work in progress. However, with this we can already see what it'll look like after. I precise: even if incomplete, this is working.

For now, this PR switched the doc blocks toggles for the following parts:

• item doc block (the one at the top)
• methods doc blocks

Some explanations on how it works: I used a trick based on checkbox. I hide the checkbox and only print its label, so when you click on the label, you check/uncheck the checkbox. Now, that also brings some downsides too:

• Positioning the label would be tricky without a parent so I had to add a div to wrap the function and its documentation. However, I see that as a positive point because we're finally "merging" the items and their documentation.
• It makes the DOM heavier because we have to add a div, a label and an input for each item with documentation.

The advantages now:

• It'll be much simpler to handle showing/hiding documentation since it'll be handled 100% with CSS. No more JS for this (except for toggling all toggles at once, this one will remain).
• A lot less of weird JS that we had.
• No more DOM manipulation when we arrive on a page to add the toggles (meaning better performance!).
• It'll finally be possible to hide/show documentation even without JS.

There are some small display differences too because currently we use width on the -/+ sign to make the whole a big larger.

Before:

After:

If this seems to be the right way to go, I'll clean up this PR (and finish to remove the remaining JS toggles) so we can finally have toggles 100% non-JS. \o/

r? @Manishearth