Merge bitcoin/bitcoin#22822: doc: Add multisig tutorial

1ef2c03452 Add multisig tutorial (lsilva01)

Pull request description:

  This PR adds a mutisig tutorial, as requested in https://github.com/bitcoin/bitcoin/issues/21278

  Although there is already a brief explanation and a functional test about the multisig implemented in https://github.com/bitcoin/bitcoin/pull/22067, this tutorial proposes to use the signet (instead of regtest), bringing the reader closer to a real environment and explaining some functions in more detail.

  I'm not sure if this format should be in this repository or on some wiki page. But as there is an open issue regarding this matter, that is my suggestion.

ACKs for top commit:
  Sjors:
    re-utACK 1ef2c03452
  prayank23:
    ACK https://github.com/bitcoin/bitcoin/pull/22822/commits/1ef2c034523d0dfc36c84186d7a110cfc7de238b

Tree-SHA512: 2c3f17a8c50e554f802029dceb28ab90a77021f135b8cbd77dca3879ba1f1a0eac6bda0afb90d1ff6b8116fb0628471687d3fb77bb255ef5d8b9590b775cbce9

doc/README.md

@@ -78,6 +78,7 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [I2P Support](i2p.md)
 - [Init Scripts (systemd/upstart/openrc)](init.md)
 - [Managing Wallets](managing-wallets.md)
+- [Multisig Tutorial](multisig-tutorial.md)
 - [PSBT support](psbt.md)
 - [Reduce Memory](reduce-memory.md)
 - [Reduce Traffic](reduce-traffic.md)