[docs] Add the project mascot image to README and documentation

[coisa]

Summary

Add the project mascot image to the repository README and documentation so the project has a more recognizable and playful visual identity.

Problem

The repository currently has a strong technical identity, but it does not yet have a visual element that gives the project a face or a bit of personality.

Scope

• add the mascot image to the repository in an appropriate tracked location
• include it in README.md
• include it in the documentation where it makes sense without overwhelming the technical content
• keep the change intentionally lightweight and presentation-focused

Expected Outcome

The repository should gain a small visual touch that makes it feel more welcoming and memorable, while keeping the current technical documentation structure intact.

Acceptance Criteria

• the mascot image is added to the repository in a stable location
• README.md displays the mascot image
• the Sphinx documentation includes the mascot image in at least one appropriate place
• the image placement feels additive and lightweight rather than decorative overload
• no unrelated branding or documentation redesign is introduced

Non-Goals

• redesigning the README structure
• redesigning the docs theme
• introducing a broader branding system
• changing command behavior, workflows, or generated outputs

Notes

This is intentionally a small personality pass for the repository, not a documentation rewrite.