Manifest: better URLs for tezt.opam

Context

This MR aims at producing the following diff for tezt.opam:

diff --git a/opam/tezt.opam b/opam/tezt.opam
index a098c5460a..d6a4431337 100644
--- a/opam/tezt.opam
+++ b/opam/tezt.opam
@@ -3,7 +3,8 @@
 opam-version: "2.0"
 maintainer: "contact@tezos.com"
 authors: ["Tezos devteam"]
-homepage: "https://www.tezos.com/"
+homepage: "https://gitlab.com/tezos/tezos/-/tree/master/tezt/lib"
+doc: "https://tezos.gitlab.io/api/odoc/_html/tezt/Tezt/index.html"
 bug-reports: "https://gitlab.com/tezos/tezos/issues"
 dev-repo: "git+https://gitlab.com/tezos/tezos.git"
 license: "MIT"

To this end, it introduces new optional arguments for targets: ?opam_homepage, ?opam_doc, and, while I was at it, ?opam_bug_reports.

I hesitated to name those arguments ?homepage, ?doc and ?bug_reports but ?doc would have been a bit confusing because ?documentation already exists…

Manually testing the MR

make -C manifest and check that the diff is empty.

https://opam.ocaml.org/doc/Manual.html#opam for the documentation of the doc clause of .opam files, which we never used up to this point. Examples can be found in ocaml/opam-repository, for instance: https://github.com/ocaml/opam-repository/blob/master/packages/alcotest-lwt/alcotest-lwt.1.6.0/opam

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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