chore: add June dates for coprogramming

[sareg0]

What this PR does

What this PR does

1. Adds upcoming Programming Co-learning dates for June and early July 2026.
2. Introduces en/calendar.md with English upcoming events.
3. Keeps German-only descriptions for those upcoming events in calendar.md (English moved to the EN calendar).
4. Localizes calendar structure in calendar.md: German section headings (Kommende / Vergangene Veranstaltungen), month section titles, and weekday labels where they were in English.
5. Points English pages to /en/calendar and removes the old /en/calendar redirect.

Why

I found myself separating the DE/EN descriptions using some text formatting (to make the two languages easier to see) and decided that maybe it's just more conventional to use the existing en folder to house the English language translations.

Before

After

[StrangeGirlMurph]

Woah the screenshots and everything are impressive! Don't worry about having to be so formal with the PRs here though.

My initial thought behind setting up the calendar with a single version under /calendar and routing /en/calendar there was that the history was already mixed and I thought it might be too much work to maintain two separate calendars and keep them in sync. Calendars are things that are more often subject to change compared to the other info. I was also considering the times where I created announcements just in english in the old pad where the event was either held in english or I just expect there to be more English speaking people who can't speak German than German speaking people who can't speak English. In that case having an extra entry in a different file to manage is a bit of a hassle.

In summary this change with the two cases mentioned above create some extra work in the day to day usage. The question is if that's worth it for us for the better user experience.

My perspective: I am willing to put in the bit of extra work for the better ux but also don't mind sticking with the current approach. I just don't have a strong opinion.

We probably have to ask more people. Maybe a small yes/no poll with a very small summary of what we're trying to decide in our internal chat would help.

If you're feeling confident in this change you can also just merge though ^^

If we end up deciding for this please also add a "Previous Events" section and within it a subsection called "Events prior to June 2026" with a link to the section of the German calendar with the old bilingual entries

Thank you!