Create doc for npm_url_dependency rule

[maxrake]

Overview

We need a standard markdown "learn more" doc for the npm URL dependency rule.

Additional context

It should follow the same format and style as the rest of the analytics docs. The doc should be called "npm_url_dependency.md" to match the (soon to be) existing published link from the rule.

Acceptance Criteria

• The "Learn More" page is hosted at https://docs.phylum.io/analytics/npm_url_dependency
• The "Issue Tags" page is updated with an entry and link to the new page