docs: refresh avatar showcase for 2xs and the six-size ladder

[nhobes]

The sizes example still said "Five sizes... from xs to xl" - 4.15 added 2xs. The example now renders the full six-step ladder starting at 2xs, with a line on where the new step earns its keep (dense rows: table cells, compact activity feeds).

Rides the unpublished 4.15.2. petal.build's avatars page inherits this via the registry at the next dep bump.

[greptile-apps]

Greptile Summary

Updates the avatar showcase to document and render the complete six-size ladder introduced in 4.15.

• Renames the example from five to six sizes.
• Explains the intended dense-layout use cases for 2xs avatars.
• Adds a 2xs avatar to the rendered size comparison.

Confidence Score: 5/5

The PR appears safe to merge.

The new 2xs value is accepted by the avatar component, has corresponding avatar styling, and is already established elsewhere in the repository’s showcases.

Important Files Changed

Filename	Overview
lib/petal_components/showcase/avatar.ex	The showcase text and markup now accurately include the supported 2xs avatar size without introducing a build or rendering defect.

_{Reviews (1): Last reviewed commit: "docs: refresh avatar showcase for 2xs an..." | Re-trigger Greptile}