DOESN'T NEED A REVIEW YET - Add user-facing documentation for custom node modules feature

[vkasanneni-nr]

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Adds user-facing documentation for the custom node modules feature for private Synthetics Job Managers.

Note: This PR doesn't need a review yet since the feature hasn't been pushed out to production yet. Will ping the docs hero once the feature is out into production.

[austin-schaefer]

Hi @vkasanneni-nr , I'm reviewing older PRs in our repo to ensure they're still valid. Is this feature still pending a release date?

[rhetoric101]

@vkasanneni-nr I am closing this since it looks out of date. Feel free to reopen it if necessary.