Accuracy badge for public ML models

Idea by @serialx.

- Let's provide "accuracy" badges for public ML models – visiting their repositories shows the accuracy value as a bdage in README (like CI build status badges)
- Feature ideas\* per-dataset / per-organization ranking
- per-commit history (i.e., how the accuracy has changed over time?)
- public dashboard to compare various open-source ML repositories

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JIRA Issue: BA-337

The issue names no files, tests, or entry points. First clarify the scope among accuracy badges, dataset or organization rankings, per-commit history, and the comparison dashboard, then identify the relevant repository entry points. Done criteria should specify which of these features is being delivered and how model accuracy is measured.

Written by the indexing model from the issue text.