Visually annotate examples that require unleased SILE‌ devel builds

[alerque]

We're successfully building examples (when necessary) using devel builds (Git HEAD at the time of site update) to showcase upcoming features, but we need to flag the examples in the showcase so people don't mistakenly thing they can use the code with only a stable tag release.

e.g. Dropcaps from #26 are currently devel only.