Consolidate Kibana personal preference documentation

## Summary

Kibana personal preference documentation is spread across feature-specific sections. Establish a canonical overview or hub that helps users discover which preferences they can set, where to set them, and whether each preference applies globally, per user, or per space.

## Existing content to audit

- [Use dark mode in Kibana](https://www.elastic.co/docs/cloud-account/dark-mode)
- [Use high-contrast mode in Kibana](https://www.elastic.co/docs/cloud-account/high-contrast)
- [Customize your navigation menu](https://www.elastic.co/docs/explore-analyze/find-and-organize/customize-navigation)
- [Manage spaces](https://www.elastic.co/docs/deploy-manage/manage-spaces), including the remembered-space login behavior added in #7418
- [Manage your Cloud account and preferences](https://www.elastic.co/docs/cloud-account)

## Proposed outcome

- Audit Kibana user-level preferences and their current documentation.
- Decide the canonical home before drafting, including whether `cloud-account/` is appropriate for content that also applies to self-managed Kibana.
- Create or improve an overview that points to the existing task pages instead of duplicating their instructions.
- Add reciprocal links so users can move between the overview and each task.
- Clarify whether each setting is user-wide or per space, and note relevant self-managed, Elastic Cloud Hosted, and Serverless differences.
- Check for other user-level preferences that belong in the overview.

## Out of scope

- Changing product behavior.
- Folding administrator-controlled space configuration or Kibana Advanced Settings into personal preferences.

## Related work

- #6937
- #7131
- #7418