Add missing navbar item and rearrange some sections
This adds an existing page to the navbar (found out in gitlab!70628 (merged)). It also adds the following changes:
- Add the missing external secrets item
- Rename Slack to ChatOps
- Move SSH keys, ChatOps, and External secrets out of the integrations section, because this didn't make sense to me. They were also buried under that section.
- Rename External integrations to External repositories integrations