Fix broken links in book (#2912)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/2889

## Additional Info

I have checked that linkcheck has succeeded on the book built locally. 👌 

```shell
$ cd book
$ mdbook serve --open
...
2022-01-14 01:13:40 [INFO] (mdbook::book): Book building has started
2022-01-14 01:13:40 [INFO] (mdbook::book): Running the html backend

$ linkcheck http://localhost:3000
Perfect. Checked 4495 links, 80 destination URLs (76 ignored).
```

Also I'll tackle running linkcheck on CI in another pull request.

book/src/faq.md

@@ -55,14 +55,10 @@ voting period the validator might have to wait ~3.4 hours for next voting
 period. In times of very, very severe network issues, the network may even fail
 to vote in new Eth1 blocks, stopping all new validator deposits!
 
-> Note: you can see the list of validators included in the beacon chain using
-> our REST API: [/beacon/validators/all](./http/beacon.md#beaconvalidatorsall)
-
 #### 2. Waiting for a validator to be activated
 
 If a validator has provided an invalid public key or signature, they will
-_never_ be activated or even show up in
-[/beacon/validators/all](./http/beacon.html#beaconvalidatorsall).
+_never_ be activated.
 They will simply be forgotten by the beacon chain! But, if those parameters were
 correct, once the Eth1 delays have elapsed and the validator appears in the
 beacon chain, there's _another_ delay before the validator becomes "active"
@@ -143,7 +139,7 @@ See [here](./slashing-protection.md#misplaced-slashing-database).
 
 If you are updating to new release binaries, it will be the same process as described [here.](./installation-binaries.md)
 
-If you are updating by rebuilding from source, see [here.](./installation-source.md#updating-lighthouse)
+If you are updating by rebuilding from source, see [here.](./installation-source.md#update-lighthouse)
 
 If you are running the docker image provided by Sigma Prime on Dockerhub, you can update to specific versions, for example:
 

book/src/intro.md

@@ -20,7 +20,7 @@ You may read this book from start to finish, or jump to some of these topics:
 - Follow the [Installation Guide](./installation.md) to install Lighthouse.
 - Learn about [becoming a mainnet validator](./mainnet-validator.md).
 - Get hacking with the [Development Environment Guide](./setup.md).
-- Utilize the whole stack by starting a [local testnet](./local-testnets.md).
+- Utilize the whole stack by starting a [local testnet](./setup.md#local-testnets).
 - Query the [RESTful HTTP API](./api.md) using `curl`.
 
 

book/src/pi.md

@@ -22,7 +22,7 @@ terminal and an Internet connection are necessary.
 
 ### 2. Install Packages
 
-Install the [Ubuntu Dependencies](installation.md#dependencies-ubuntu).
+Install the [Ubuntu Dependencies](installation-source.md#ubuntu).
 (I.e., run the `sudo apt install ...` command at that link).
 
 > Tips:

book/src/redundancy.md

@@ -86,7 +86,7 @@ now processing, validating, aggregating and forwarding *all* attestations,
 whereas previously it was likely only doing a fraction of this work. Without
 these flags, subscription to attestation subnets and aggregation of
 attestations is only performed for validators which [explicitly request
-subscriptions](subscribe-api).
+subscriptions][subscribe-api].
 
 There are 64 subnets and each validator will result in a subscription to *at
 least* one subnet. So, using the two aforementioned flags will result in

book/src/validator-inclusion.md

@@ -98,7 +98,7 @@ curl -X GET "http://localhost:5052/lighthouse/validator_inclusion/0/global" -H
 Returns a per-validator summary of how that validator performed during the
 current epoch.
 
-The [Global Votes](#consensusglobal_votes) endpoint is the summation of all of these
+The [Global Votes](#global) endpoint is the summation of all of these
 individual values, please see it for definitions of terms like "current_epoch",
 "previous_epoch" and "target_attester".