chore: add workflow linting paths trigger

[MikeMcC399]

Description

For the workflows using actions/setup-node that only trigger on certain pull_request paths:

• .github/workflows/doctoc.yml
• .github/workflows/markdown-link-check.yml

add the workflow's own path to the pull_request event trigger.

Motivation and Context

#2565 proposes a major version update to the GitHub JavaScript Action actions/setup-node from 6.4.0 to 7.0.0. This did not trigger any CI run in:

• .github/workflows/doctoc.yml
• .github/workflows/markdown-link-check.yml.

No change is needed to the following workflow, since this is already triggered by any pull request:

• .github/workflows/eclint.yml

Testing Details

Check the workflow logs for the following, and confirm that they were each triggered by this PR:

• https://github.com/nodejs/docker-node/actions/workflows/doctoc.yml
• https://github.com/nodejs/docker-node/actions/workflows/eclint.yml
• https://github.com/nodejs/docker-node/actions/workflows/markdown-link-check.yml

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.