Add staging.crates.io-index repository under automation

[Kobzol]

Repo: https://github.com/rust-lang/staging.crates.io-index

Modeled after #1306.

CC @Turbo87

Extracted from GH:

org = "rust-lang"
name = "staging.crates.io-index"
description = "Registry index for staging.crates.io"
bots = []

[access.teams]
security = "pull"
crates-io = "write"
crates-io-on-call = "write"

[access.individuals]
jdno = "admin"
pietroalbini = "admin"
tshepang = "write"
carols10cents = "write"
Xylakant = "write"
LawnGnome = "write"
eth3lbert = "write"
listochkin = "write"
Rustin170506 = "write"
jtgeibel = "write"
Turbo87 = "write"
mdtro = "write"
skade = "write"
rust-lang-owner = "admin"
MarcoIeni = "admin"
Mark-Simulacrum = "admin"
justahero = "write"
Veykril = "write"

[[branch-protections]]
pattern = "master"
required-approvals = 0
pr-required = false

[Turbo87]

if it works for prod, it should also work for staging. should've probably done this on staging first and then on prod... 😂