Add a new category Sliding Window and also include the question on longest substring with k unique characters.

Hi, I would like to contribute to the project by making a new folder for sliding window technique. I would also like to add a question
on the same.
Please assign this issue to me and add Hacktoberfest label to it.

No specific files or tests are named. Start by inspecting the repository's existing category folders and question organization, then determine where the new Sliding Window category belongs. Done means the category exists and includes the requested longest-substring-with-k-unique-characters question in the project's established format.

Written by the indexing model from the issue text.