docs: add cli exec examples

[AkashKumar7902]

Summary

• add a dedicated local pipeline execution guide for woodpecker-cli exec
• link local execution from the welcome page and first-pipeline tutorial
• document backend selection, metadata overrides, environment variables, and secrets

Closes #6520

Validation

• GOROOT=$(go env GOROOT) GOTOOLCHAIN=local PATH="$(go env GOROOT)/bin:$PATH" go run -tags test ./cmd/cli --disable-update-check exec --backend-engine dummy --pipeline-event pull_request --commit-branch main --repo woodpecker-ci/woodpecker .woodpecker/docs.yaml
• GOROOT=$(go env GOROOT) GOTOOLCHAIN=local PATH="$(go env GOROOT)/bin:$PATH" go run -tags test ./cmd/cli --disable-update-check exec --backend-engine dummy --pipeline-event pull_request --env GOFLAGS=-mod=readonly --secrets deploy_token=example .woodpecker/docs.yaml
• cd docs && pnpm format:check
• GOROOT=$(go env GOROOT) GOTOOLCHAIN=local PATH="$(go env GOROOT)/bin:$PATH" make build-docs
• make spellcheck

[woodpecker-bot]

Account with some automation detected 🤖🧑‍💻

@AkashKumar7902

We noticed a mix of typical human activity and some patterns often associated with automation or AI agents.

Human? 65% likely to be a human.

Signal	Points	Detail
Automated branch/PR workflow	+35	22/27 branch creations followed by PRs within 71s

Have thoughts about this account? Let the community know.

_{🕵🏽 Analyzed 192 public events via AgentScan}

[woodpecker-bot]

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