Prepare for Netbox 4.6

[tobiasge]

Related Issue: -

New Behavior

• Netbox 4.6 now builds it's documentation with zensical (Closes #21635: Migrate from mkdocs to Zensical netbox#21742)
• Use Ubuntu 26.04 as base image

Contrast to Current Behavior

• Documentation builds with Netbox 4.6 as intended by the Netbox team

Discussion: Benefits and Drawbacks

• Keeps our base image updated with the newest Ubuntu

Changes to the Wiki

• Update sites mentioning Ubuntu und mkdocs

Proposed Release Note Entry

• Compatibility with Netbox 4.6
• New Ubuntu 26.04 base image
• With that our Python version is now 3.14.x

Double Check

• I have read the comments and followed the PR template.
• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.

[cimnine]

Lgtm

[tobiasge]

Images for main and the current release are expected to fail here.
Merging this to develop that we have images for feature and pre-release with the new base image.