Streamline retroactive compat bounds adjustment

- [x] Document the compatibility specification format
- https://pkgdocs.julialang.org/v1/registries/#Registry-Compat.toml
- [x] Add CI checks to verify that specifications are properly formatted (and only run checks on edited files)
- https://github.com/JuliaRegistries/RegistryCI.jl/pull/575
- [ ] Add instructions for retroactive compat bounds adjustment to README and link exemplary merged PRs
- [ ] Add some highly automated `@JuliaRegistrator set_compat_bounds(...)` invocation
- [ ] Moved to separate issue: https://github.com/JuliaRegistries/Registrator.jl/issues/436

(the 4th item is likely excessive)

Start with the repository README and the linked Registry Compat.toml specification. Review existing merged pull requests that adjusted compatibility bounds, then document the retroactive adjustment procedure and link representative examples; the README should clearly explain the required format and workflow.

Written by the indexing model from the issue text.