docs: Prevent image overflow on quick tutorial page

[dedyf5]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

This PR fixes a layout issue on the "Quick Tutorial" page (https://thanos.io/tip/thanos/quick-tutorial.md/) where the architecture diagrams for Sidecar and Receive were overflowing the main content container.

• The images are now constrained within their parent container.
• This ensures the diagrams scale down correctly with the viewport, preventing horizontal scrolling and maintaining the page layout.

Verification

Before:
(The image overflows the content area, requiring horizontal scroll.)

After:
(The image now scales down correctly within the content area.)