sprints page liniks show generic anchors instead of readable slugs

On the sprints page, TOC links show generic anchors instead of readable ones
- expected: https://ep2026.europython.eu/sprints/#open-source-projects
- actual: https://ep2026.europython.eu/sprints/#heading-3

_IMPORTANT_: we have already linked https://ep2026.europython.eu/sprints/#heading-3 in the email draft to attendees. So we might want to delay fixing the problem, unless it can be done before the email is sent out hopefully later today, or if we add a hidden backward-compat anchor (id="heading-3") alongside the new slug.