Set up mentorship team "properly"

To facilitate getting out a blog post, we added the mentorship
team (previously chartered under the name "internship team") under the
name "mentorship-programs".  Let's now make some edits to set this up
more "properly".

First, let's rename it to just "mentorship".  Probably I understand
the hesitancy to claim such a broad mandate by calling it that rather
than "mentorship-programs", but it just seems unlikely to me that we'd
ever have both a "mentorship-programs" team and a "mentorship" team,
so we should just use the shorter name.  (My guess is that if we ever
wanted some other kind of mentorship, it's more likely that this team
would expand its mandate than that we'd add a second team.)  If that
really makes us uncomfortable, then probably I'd suggest going back to
the "internship" name, as that is actually rather unambiguous here.

Second, as we discussed on the chartering thread, let's not include
the mentors as members of the mentorship team and instead break that
out into a separate marker team.  It's good for teams to have a clear
charter and purpose for which the members are responsible.  If there
really is a divide where the mentors aren't responsible for the work
stated in the charter of organizing and administering this program,
then the membership should be split out.  That is, we should ask, "who
would be on an FCP for a given decision?"  Having such clarity, e.g.,
helps the council in cleanly delegating matters to teams, which we
prefer doing, rather than having to specify a delegation to just the
leads.

In this PR, that marker team is marked as a subteam of T-mentorship.
I don't see us doing this anywhere else in this repository -- making a
marker team a subteam -- but neither do I see it documented as not
working, and it seems appropriate in this case, so we'll see if it
passes CI.

Third, let's set up all the other normal accoutrements of such a team,
including an entry on the `rust-lang.org` website for the team, a
repository for the team's charter and whatnot, etc.

repos/rust-lang/mentorship-team.toml

@@ -0,0 +1,7 @@
+org = "rust-lang"
+name = "mentorship-team"
+description = "Home of the Rust mentorship team"
+bots = ["rustbot", "rfcbot"]
+
+[access.teams]
+mentorship = "maintain"

teams/mentorship-programs.toml → teams/mentors.toml

@@ -1,11 +1,9 @@
-name = "mentorship-programs"
-subteam-of = "launching-pad"
+name = "mentors"
+kind = "marker-team"
+subteam-of = "mentorship"
 
 [people]
-leads = [
-    "Kobzol",
-    "jackh726"
-]
+leads = []
 members = [
     "Kobzol",
     "jackh726",

teams/mentorship.toml

@@ -0,0 +1,30 @@
+name = "mentorship"
+subteam-of = "launching-pad"
+
+[people]
+leads = [
+    "Kobzol",
+    "jackh726"
+]
+members = [
+    "Kobzol",
+    "jackh726",
+]
+alumni = []
+
+[[github]]
+orgs = ["rust-lang"]
+
+[rfcbot]
+label = "T-mentorship"
+name = "Mentorship"
+ping = "rust-lang/mentorship"
+
+[website]
+name = "Mentorship team"
+description = "Manages involvement of Rust in mentorship and internship programs"
+repo = "http://github.com/rust-lang/mentorship-team"
+zulip-stream = "gsoc"
+
+[[zulip-groups]]
+name = "T-mentorship"