Remove the redundant broken gh-pages deploy job

The repo has two workflows publishing to the same gh-pages branch:

  main-docs.yml     JamesIves/github-pages-deploy-action + the automatic
                    GITHUB_TOKEN                              -> works
  documentation.yml webfactory/ssh-agent + secrets.GH_PAGES_DEPLOY
                                                              -> fails

The second has been failing on every push to main with

    git@github.com: Permission denied (publickey)

because the GH_PAGES_DEPLOY key is no longer valid. It has gone unnoticed
because the site does still deploy - the other workflow does it, and needs
no configured secret.

Rather than rotate the key and end up with two workflows racing to publish
the same content, this deletes the gh-release job. documentation.yml
becomes PR validation only; main-docs.yml remains the single deploy path.
GH_PAGES_DEPLOY is now referenced nowhere and the repo secret can be
deleted.

This also removes a hardcoded git identity pointing at a former
contributor's personal email address.

Other fixes while in these files:

- main-docs.yml triggered on "push" with no branch filter, and its Deploy
  step had no condition, so a push of any branch to this repo would
  publish that branch's docs over the live site. Now scoped to main.
- Dropped its unused "python-version: [3.6]" matrix. The job only runs
  yarn and npm; the value did nothing but label the check "(3.6)".
- Pinned Node 20 explicitly in both workflows. Docusaurus 3 requires
  >= 20 and main-docs.yml was relying on the runner default, which is the
  same implicit dependency that left documentation.yml stuck on Node 14.
- Switched main-docs.yml to "yarn install --frozen-lockfile", matching
  documentation.yml, so the deploy builds what the lockfile pins.
- Dropped "sudo apt-get install -y yarn"; yarn is preinstalled on
  ubuntu-latest and the apt package is a different tool.
- checkout/setup-node v1 and v2 -> v4, clearing the deprecation warnings.

The deploy action itself is left at JamesIves v3 deliberately. v4 renames
its inputs, and this is the only working deploy path, so it is not
something to change in the same PR that touches everything around it.

Verified locally on Node 20: "yarn install --frozen-lockfile" succeeds
against the committed lockfile and the build produces 50 pages.

Author: caglarpir

.github/workflows/documentation.yml

@@ -1,19 +1,20 @@
 name: documentation
 
+# Validates that the docs site still builds. Deployment lives in
+# main-docs.yml, which publishes to gh-pages using the automatic
+# GITHUB_TOKEN and needs no configured secret.
 on:
   pull_request:
     branches: [main]
-  push:
-    branches: [main]
 
 jobs:
   checks:
-    if: github.event_name != 'push'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
-      - uses: actions/setup-node@v1
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
         with:
+          # Docusaurus 3 requires Node >= 20
           node-version: '20.x'
       - name: Test Build
         run: |
@@ -26,30 +27,3 @@ jobs:
             npm i
           fi
           npm run build
-  gh-release:
-    if: github.event_name != 'pull_request'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - uses: actions/setup-node@v1
-        with:
-          node-version: '20.x'
-      - uses: webfactory/ssh-agent@v0.5.0
-        with:
-          ssh-private-key: ${{ secrets.GH_PAGES_DEPLOY }}
-      - name: Release to GitHub Pages
-        env:
-          USE_SSH: true
-          GIT_USER: git
-        run: |
-          cd docs
-          git config --global user.email "saifulislam84210@gmail.com"
-          git config --global user.name "Saif Ul Islam"
-          if [ -e yarn.lock ]; then
-            yarn install --frozen-lockfile
-          elif [ -e package-lock.json ]; then
-            npm ci
-          else
-            npm i
-          fi
-          npm run deploy

.github/workflows/main-docs.yml

@@ -1,23 +1,29 @@
-on: [push]
+name: deploy documentation
+
+# Builds the docs site and publishes it to the gh-pages branch, which is what
+# GitHub Pages serves. Authenticates with the automatic GITHUB_TOKEN, so there
+# is no deploy key or secret to maintain.
+on:
+  push:
+    branches: [main]
 
 jobs:
   build_docs_job:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        python-version: [3.6]
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
-    - name: Dependencies
-      run: |
-        sudo apt-get install -y yarn
-      id: build
+      uses: actions/checkout@v4
+    - name: Set up Node
+      uses: actions/setup-node@v4
+      with:
+        # Docusaurus 3 requires Node >= 20. This was previously left to the
+        # runner default, which happened to be new enough but was not pinned.
+        node-version: '20.x'
     - name: Build the Website
       run: |
         cd docs
-        yarn
-        npm run build  
+        yarn install --frozen-lockfile
+        npm run build
     - name: Deploy
       uses: JamesIves/github-pages-deploy-action@releases/v3
       with: