Document how to use a local network in the CI

[Thoralf-M]

Projects that want to build on IOTA often want to have a CI that spawns a localnet so one isn't rate limited by the public faucet or similar things.
Not sure where in the docs it would be put best, but we should have an example for that, also for the gas station additionally and potentially even more things?
In iotaledger/iota-rust-sdk#260 an example can be seen with the gas station, localnet was already added in another PR before, but is visible in the same file

[Thoralf-M]

Here is a script for how to start localnet + gas station locally: iotaledger/iota-rust-sdk#306