Remove links to old master branch (#2190)

## Proposed Changes

In preparation for deleting the `master` branch, remove all links to it from the book/README.

book/src/contributing.md

@@ -14,7 +14,7 @@ to work on.
 
 To start contributing,
 
-1. Read our [how to contribute](https://github.com/sigp/lighthouse/blob/master/CONTRIBUTING.md) document.
+1. Read our [how to contribute](https://github.com/sigp/lighthouse/blob/unstable/CONTRIBUTING.md) document.
 2. Setup a [development environment](./setup.md).
 3. Browse through the [open issues](https://github.com/sigp/lighthouse/issues)
    (tip: look for the [good first

book/src/intro.md

@@ -30,7 +30,7 @@ to understand how we develop and test Lighthouse.
 ## About this Book
 
 This book is open source, contribute at
-[github.com/sigp/lighthouse/book](https://github.com/sigp/lighthouse/tree/master/book).
+[github.com/sigp/lighthouse/book](https://github.com/sigp/lighthouse/tree/unstable/book).
 
-The Lighthouse CI/CD system maintains a hosted version of the `master` branch
+The Lighthouse CI/CD system maintains a hosted version of the `unstable` branch
 at [lighthouse-book.sigmaprime.io](http://lighthouse-book.sigmaprime.io).

book/src/local-testnets.md

@@ -4,5 +4,5 @@ During development and testing it can be useful to start a small, local
 testnet.
 
 The
-[scripts/local_testnet/](https://github.com/sigp/lighthouse/tree/master/scripts)
+[scripts/local_testnet/](https://github.com/sigp/lighthouse/tree/unstable/scripts)
 directory contains several scripts and a README that should make this process easy.

book/src/mainnet-validator.md

@@ -4,7 +4,7 @@
 [lh-book]: https://lighthouse-book.sigmaprime.io/
 [testnet-validator]: ./testnet-validator.md
 [advanced-datadir]: ./advanced-datadir.md
-[license]: https://github.com/sigp/lighthouse/blob/master/LICENSE
+[license]: https://github.com/sigp/lighthouse/blob/stable/LICENSE
 [slashing]: ./slashing-protection.md
 [discord]: https://discord.gg/cyAszAh
 

book/src/slashing-protection.md

@@ -177,4 +177,4 @@ out on Discord if you need help.
 The Lighthouse developers do not guarantee the perfect functioning of this software, or accept
 liability for any losses suffered. For more information see the [Lighthouse license][license].
 
-[license]: https://github.com/sigp/lighthouse/blob/master/LICENSE
+[license]: https://github.com/sigp/lighthouse/blob/stable/LICENSE