Create doc for `eval_calls` rule

[joephylum]

Overview

The EvalCalls rule is getting put back into prod and will need an accompanying docs page.

Additional context

It should follow the same format and style as the rest of the analytics docs. The doc should be called "eval_calls.md".

Acceptance Criteria

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