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Implement compat checks and bump compat level
There's currently [apkg.compat](https://pkg.labs.nic.cz/pages/apkg/config/#apkgcompat) config option specifying `apkg` compat level. - [x] Implement mechanism to compare `apkg`'s compat level with project specified compat level and emit warnings/hints when appropriate. - [x] Create new compat doc page describing individual compat levels - [x] bump compat to 2 for %"v0.3" as there are notable changes in `master`
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