Synchronize project metadata with GitHub

### Goal
Periodically synchronize metadata for each downstream project with its GitHub repository.

### Motivation
Enhanced integration with GitHub, long-term host of all downstream OSS projects:
- [x] More **informative GitHub layout** thanks to repository descriptions
- [x] **Better navigation**: automatic project URLs (my suggestion is RTD links whenever possible—from the README docs badge?)
- ~~**Community flow**: Discord webhooks~~ tracked in jaraco/pytest-perf#11
- [x] **GitHub Topics** (those under the website URL—from Trove classifiers?)

@jaraco Have I included everything? 😀

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Derived from discussions @ jaraco/skeleton#208.

No files, tests, or entry points are named. Start by reviewing the discussion in jaraco/skeleton#208 and the existing project metadata and GitHub integration, then define how descriptions, URLs, and topics should be synchronized while excluding the Discord webhook work. Done means the agreed metadata is periodically synchronized for downstream projects.

Written by the indexing model from the issue text.