Update README and add a summary of translation progress

[delucis]

Description (required)

• Generates a summary.svg file as part of Lunaria builds that shows progress for each locale.
• Adds this with a link to our Lunaria dashboard to the README.

Notes

• To support reasonable display on both light and dark backgrounds, this uses a somewhat muted grey text colour. This colour has roughly a 10:1 contrast ratio against both black and white.
• On mobile screens the text will appear slightly small, but I tried to strike a balance between a decent display on desktop and mobile.
• We only build Lunaria on main so can’t really preview this PR but here are a few examples:

Example output

Example in context in README.md

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[sarah11918]

This looks lovely!

[TheOtterlord]

LGTM!

We could revisit with an image each for dark/light if we want more contrast for certain themes (https://github.blog/developer-skills/github/how-to-make-your-images-in-markdown-on-github-adjust-for-dark-mode-and-light-mode/#one-snippet-two-themes)

[delucis]

Thanks! Yeah, we can consider light/dark support in the future if we want.