Do inventory of missing docs

Go through all Sigsum repos and see what essential docs are missing, create issues and add the ones that should be fixed ASAP in https://git.glasklar.is/groups/sigsum/-/milestones/14#tab-issues. Ranges from things like "there should be a LICENSE file" to "how would I even get started with my own Sigsum use-case".

Any input most welcome on what you're missing.