[DOCS] Skip docs-build-pr CI for changelogs and release bundles

[lcawl]

Summary

This PR updates the docs-build workflow such that when a PR only changes docs/changelog/ or docs/releases files (such as in #275684), the CI does not run.
The workflow is skipped only when all changed files match that ignored path; if the PR also changes anything outside those folders, it still runs.

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
- [ ] Documentation was added for features that require explanation or tutorials
- [ ] Unit or functional tests were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
- [ ] This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
- [ ] Flaky Test Runner was used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
- [ ] Review the backport guidelines and apply applicable backport:* labels.

Identify risks

N/A

[infra-vault-gh-plugin-prod]

Pinging @elastic/experience-docs (Team:Docs)

[kibanamachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 2107be7

[lcawl]

The better long-term solution is in elastic/docs#3343 so I'll close this one