add a conf toggle so sponsors->prospectus in nav

[phette23]

Sets the toggle to true so this takes effect right away. I debated if we should rename the menu link to "Prospectus" but I actually think it's better / clearer to leave it as "Sponsors", though that's debatable.

[triplingual]

@phette23 Is this PR moot at this point?

[phette23]

It's irrelevant for this year now but it'd be useful for future years. If you look at the progress of sponsorships:

1. We have no sponsors <--- useful here, /sponsors/ links to prospectus instead of an empty sponsors page
2. We have some sponsors — we want to show the sponsors page & it links to prospectus (current state)
3. We're no longer accepting sponsors — no need to link to prospectus, site.data.conf.sponsor-buttons: false

[triplingual]

Ah, understood. Would it be risky to merge it now? Do you want to wait until after the conference for any other reason?
Probably just my slightly compulsive but irregularly present desire for neatness kicking in.

[phette23]

I don't think there's any harm but I added a commit just now so sponsors-links-to-prospectus is false.