chore: disable terraform-docs push when on forks and sign commit

[d-costa]

what

• Disable terraform-docs push when on forks
• Use ghcommit-action to create a verified commit and push the changes, instead of using terraform-docs' git-push, which does not sign commits
• Set continue-on-error to true since we only commit in the next step
• Add instructions to run tfdocs locally for forks in workflow summary

why

• The ci workflow fails when running on forks because it fails to push changes (as it should): https://github.com/runatlantis/terraform-gce-atlantis/actions/runs/8737454606/job/24044031175
• The commits being signed/verified are really useful when people's forks live inside organizations that require signed commits (rebasing and pushing unsigned commits in the fork are blocked)

references

• https://github.com/planetscale/ghcommit-action
• Enable Verified Commits (with GPG key as secret) terraform-docs/gh-actions#63 (comment)
• Fix pinning of machine_type #149 (comment)
• Example run (pushes a commit, and status is failed): https://github.com/runatlantis/terraform-gce-atlantis/actions/runs/8760478846/job/24045502598

[bschaatsbergen]

LGTM 🚀 nice work @d-costa