Link pattern names to theme pattern partial

It would be convenient, and possibly helpful during development, to be able to jump right from the name of the pattern to the relevant file in the repo when browsing a pattern library on GitHub. This should be accomplishable with a relative URL, since the generator action already knows the theme

Hypothetical change,

Markup Before | Markup After
--------- | ------
`mytheme/card-call-to-action` | `[mytheme/card-call-to-action](../../themes/mytheme/patterns/card-call-to-action.php)`

Start by locating the generator action that renders pattern names and inspect how it derives the theme and pattern paths. Update the generated markup so each pattern name links to the corresponding relative PHP partial path shown in the example, then verify the output against the before-and-after markup.

Written by the indexing model from the issue text.