Gallery page: submit your own DevCard via PR

[chiruu12]

Create a gallery section in the README where users can submit their own DevCard SVGs via pull request.

How it works:

1. User generates their card: devcard generate <username> --format svg --theme dark -o gallery/<username>.svg
2. User opens a PR adding their SVG to gallery/
3. PR gets merged, their card appears in the README gallery

This creates a contribution loop — every submission is a star + a PR + social proof.

Add instructions to CONTRIBUTING.md for how to submit your card.