docs: Improved and consistent diagrams

[a-thaler]

Description

Changes proposed in this pull request (what was done and why):

• Tried to apply one consistent theme to all diagrams
• Improved the diagrams in general

Changes refer to particular issues, PRs or documents:

Traceability

• The PR is linked to a GitHub issue.
• The PR has a milestone set.
• The PR has a respective area and kind label.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.

[NHingerl]

• Across all diagrams, choose either 3rd person or infinitive verbs; don't mix approaches.
• Many of the text bits are outdated; the newest version is in chore: Sync markdown to DITA, volume 2 #1255.

[a-thaler]

The outdated text I will update as soon as the other PR is merged

[NHingerl]

The lonely logo floating in whitespace looks weird

[NHingerl]

If you got rid of the Kyma logo in the reference key, you can also remove it from the diagrams. Saves space, especially in the small Istio boxes.

[NHingerl]

If you got rid of the Kyma logo in the reference key, you can also remove it from the diagrams. Saves space, especially in the small Istio boxes.

Apart from that, 🟢