Link every pattern page to its own lesson in the course

[design-gurus]

Each pattern page's Go deeper section now opens with a direct link to that pattern's introduction lesson in the course player, above the existing link to the course landing page.

## Go deeper

- This pattern's introduction in the course: [Introduction to Sliding Window Pattern](https://www.designgurus.io/course-play/grokking-the-coding-interview/doc/introduction-to-sliding-window-pattern)
- The problems that use it, with worked solutions in six languages and runnable tests: [Grokking the Coding Interview](https://www.designgurus.io/course/grokking-the-coding-interview)

All 41 slugs come from a live get_course dump rather than being guessed from the pattern name, and every URL was checked for a 200 before anything was written. Four of them would have been wrong if guessed: introduction-to-stack, introduction-to-hash-tables, introduction-to-graph, and introduction-prefix-sum-pattern, which is missing the word "to".

Tree Level Order Traversal deliberately points at introduction-to-tree-breadth-first-search-pattern. That is the surviving slug from the chapter 12 and 13 merge, and moving it would 404 a working URL. The link text uses the current lesson title, so the mismatch is invisible to a reader.