Add synopsis to `octez-libs` sub-libraries

The following discussion from !8637 (merged) should be addressed:

• @raphael-proust started a discussion:

  Suggestion for a future improvement / follow-up issue:

  Use the synopsis to

  a. generate a long-form description of the main package (something like "this package contains multiple base libraries for the octez project\n\ntezos-stdlib: yet-another local-extension of the OCaml standard library\ntezos-error-monad: error monad\n…")

  b. add descriptions in the generated documentation index (something like Format.fprintf pp "- {{!module-%s}%s}: %s" registered registered synopsis)