docs: Add a more formal specification of the LogStore format

The new file is meant to be a short specification of the format, one
that describes the format, and not how the code is parsing various
parts of it.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
logstore: Add a link to the spec from the docs

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
logstore.codec: Document that xfrm-info records are supported too

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
logstore: Update the docs, chunk hashes are checked nowadays

Signed-off-by: Gergely Nagy <algernon@balabit.hu>