Document how the hw card stack is supposed to work and interoperate

As mentioned on IRC the other day, and out of what seems a bit of a recurring issue, I think it would be nice to document how the hw card stack is supposed to work and interoperate with the various backend implementations. While ideally this all should work transparently, this does not seem to be the case currently. This also would help packagers decide what to depend/recommend/suggest for the packaging metadata.

I think it would be nice to also document what's the preferred (from a Sequoia PoV) stack to use (say pcscd or scdaemon, etc), perhaps depending on the circumstances. For example if the user wants to work with Sequoia and GnuPG in equal terms then probably using gpg-agent + scdaemon is the better option, but if the user wants to use only Sequoia then pcscd is perhaps the preferred choice (?).

It would also be nice to cover or warn about potential interoperability issues with different stacks (while those would be solved or documented by their respective projects, in the end this will affect Sequoia users, so I think it's probably worth at least to mention any such known issues), for example that running in parallel scdaemon and pcscd can end up fighting for access to the hw card, and then perhaps how to configured them so that they can share access.