docs: restructure problem paths, change section display to groups, and improve documentation content

apps/docs/content/(01-linear-structures)/hash-table/index.mdx

@@ -39,10 +39,12 @@ def get_counter(arr: list[int]) -> dict[int, int]:
     return counter
 ```
 
-::: tip[Python provides a built-in `collections.Counter` class]
+:::tip
+Python provides a built-in [`collections.Counter`](https://docs.python.org/3/library/collections.html#collections.Counter) class that does this for us.
+:::
+
 ```python
 from collections import Counter
 
 fq = Counter(arr)
 ```
-:::