[PR4061][thesis] doc/designs: Add a design page for application-specific passwords

[Richard Kalinec]

Good evening,

I am a student of Faculty of Informatics of Masaryk University working
on my master thesis with Red Hat. As a core of this thesis, I aim to
extend FreeIPA (and SSSD) to allow multiple passwords for a single
user (application-specific passwords) and ideally also some form of
permissions management for them.

For more information, see issue #4510: https://pagure.io/freeipa/issue/4510

I already created a PR #4061 with an initial design page:
https://github.com/freeipa/freeipa/pull/4061
The design page is, however, far from complete, and everything needs
to be carefully thought through and analyzed. Mr. Simo Sorce already
provided valuable feedback; more feedback and discussion is highly
appreciated.

Best regards,
Richard Kalinec