feat: adds zk linkcheck to ci to prevent dead links (matter-labs#870)

## What ❔
- Adds `zk linkcheck` to CI 
- Adds `zk linkcheck` to Pull request template
<!-- What are the changes this PR brings about? -->
<!-- Example: This PR adds a PR template to the repo. -->
<!-- (For bigger PRs adding more context is appreciated) -->

## Why ❔
- Prevents dead links from being introduced to repo
<!-- Why are these changes done? What goal do they contribute to? What
are the principles behind them? -->
<!-- Example: PR templates ensure PR reviewers, observers, and future
iterators are in context about the evolution of repos. -->

## Checklist

<!-- Check your PR fulfills the following items. -->
<!-- For draft PRs check the boxes as you complete them. -->

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zk fmt` and `zk lint`.
- [x] Spellcheck has been run via `zk spellcheck`.

.github/pull_request_template.md

@@ -19,3 +19,4 @@
 - [ ] Documentation comments have been added / updated.
 - [ ] Code has been formatted via `zk fmt` and `zk lint`.
 - [ ] Spellcheck has been run via `zk spellcheck`.
+- [ ] Linkcheck has been run via `zk linkcheck`.

.github/workflows/check-links.yml

@@ -0,0 +1,41 @@
+name: Check Links
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  merge_group:
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  linkcheck:
+    runs-on: [matterlabs-ci-runner]
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3
+        with:
+          submodules: "recursive"
+      - name: Use Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version: 18
+
+      - name: Setup environment
+        run: |
+          echo ZKSYNC_HOME=$(pwd) >> $GITHUB_ENV
+          echo $(pwd)/bin >> $GITHUB_PATH
+          echo IN_DOCKER=1 >> .env
+
+      - name: Start services
+        run: |
+          docker compose up -d zk
+
+      - name: Build zk
+        run: |
+          ci_run zk
+
+      - name: Run zk linkcheck
+        run: |
+          ci_run zk linkcheck

docs/specs/prover/boojum_function_check_if_satisfied.md

@@ -12,7 +12,7 @@ variables circuit columns that are under PLONK copy-permutation constraints (so
 in programming languages), and the witness ephemeral values that can be used to prove certain constraints, for example
 by providing an inverse if the variable must be non-zero.
 
-![Check_if_satisfied.png](./img/boojum_function_check_if_satisfied/check_if_satisfied.png)
+![Check_if_satisfied.png](./img/boojum_function_check_if_satisfied/Check_if_satisfied.png)
 
 Next we prepare a view. Instead of working with all of the columns at once, it can be helpful to work with only a
 subset.

docs/specs/prover/overview.md

@@ -1,7 +1,7 @@
 # Intro to zkSync’s ZK
 
 This page is specific to our cryptography. For a general introduction, please read:
-[https://docs.zksync.io/userdocs/intro/#introduction](https://docs.zksync.io/userdocs/intro/#introduction)
+[https://docs.zksync.io/build/developer-reference/rollups.html](https://docs.zksync.io/build/developer-reference/rollups.html)
 
 As a ZK rollup, we want everything to be verified by cryptography and secured by Ethereum. The power of ZK allows for
 transaction compression, reducing fees for users while inheriting the same security.