Unify xbps/repositories/signing.md with void-packages/README.md?

[camoz]

#641 made me wonder if it's better to drop the section Signing Repositories and link to the README instead. Alternatively, drop the section in the README and link to the docs. Goal: not having duplicate documentation, which will probably lead to divergence in the future again.

But both "solutions" would make one document less self-contained... I don't really like either of them. But right now the README seems more of a "start to finish" guide without many links, so it's probably better to link from the docs to the README?

Another wild idea: Include the whole README into the docs, somewhere below 4. XBPS Package Manager. Not sure if that makes sense...

Maybe I'm just overthinking.