Remove the redundant broken gh-pages deploy job

[caglarpir]

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

Workflow	Job	Auth	Result
main-docs.yml	build_docs_job	JamesIves/github-pages-deploy-action + automatic GITHUB_TOKEN	works — this is what is live
documentation.yml	gh-release	webfactory/ssh-agent + secrets.GH_PAGES_DEPLOY	fails

The second fails on every push to main:

git@github.com: Permission denied (publickey)
fatal: Could not read from remote repository.

The GH_PAGES_DEPLOY key is no longer valid. This has gone unnoticed because the site does still deploy — the other workflow does it, and it needs no configured secret. gh-pages was last updated today, and mapillary.github.io currently serves the Docusaurus 3 build from #186.

Rotating the key would fix a job that is redundant, leaving two workflows racing to publish the same content. So this deletes the gh-release job instead. documentation.yml becomes PR validation only; main-docs.yml stays the single deploy path.

GH_PAGES_DEPLOY is now referenced nowhere and the repo secret can be deleted.

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

Other fixes in the same files

Three of these are more than cosmetic:

• main-docs.yml deployed from any branch. It triggered on push with no branch filter, and the Deploy step had no condition — so pushing any branch to this repo would publish that branch's docs over the live site. Now scoped to main. (I hit this myself: pushing a test branch to my fork triggered a deploy attempt there.)
• Node was unpinned in main-docs.yml. Docusaurus 3 requires >= 20; the job relied on whatever the runner happened to ship. That is the same implicit dependency that left documentation.yml stuck on Node 14 until Upgrade Docusaurus 2.0.0-beta.5 → 3.10.2 #186. Both are now pinned to 20.x.
• main-docs.yml used bare yarn, not --frozen-lockfile, so the deploy could resolve something other than what the lockfile pins. Now matches documentation.yml.

Smaller:

• Dropped the unused python-version: [3.6] matrix — the job only runs yarn/npm, so it did nothing but label the check (3.6).
• Dropped sudo apt-get install -y yarn; yarn is preinstalled on ubuntu-latest, and the apt package of that name is a different tool.
• checkout/setup-node v1/v2 → v4, clearing the Node 20 deprecation warnings.

Deliberately not changed

The deploy action stays at JamesIves/github-pages-deploy-action@releases/v3. v4 renames its inputs, and this is the only working deploy path — not something to change in the same PR that touches everything around it. Worth a follow-up on its own.

Test plan

• Both workflow files validated as YAML; triggers and jobs confirmed
• grep secrets. across .github/workflows/ returns only the automatic GITHUB_TOKEN
• Locally on Node 20: yarn install --frozen-lockfile succeeds against the committed lockfile and the build produces 50 pages
• The checks job on this PR exercises the edited documentation.yml. main-docs.yml only runs on push to main, so its first real exercise is the merge — the change there is small and the build command is the same one checks runs.