Create 0025-reverse-nodes-in-k-groups-hard.md

Change Summary

Added the solution for the LeetCode problem: 0025: Reverse Nodes in K groups

If you haven't fulfilled the below requirements or even delete the entire checklist, your PR won't be reviewed and will be closed without notice.

[x] This Pull Request is all my own work. (You'll be blacklisted if you are caught for plagiarism.)
[x] I've read CONTRIBUTING.md
[x] I've applied LaTex for all variables, formulas and time / space complexity instead of using backticks
[x] I've started the app locally and verified all the content and all links (if applicable) are accessible correctly
[x] I've included Complexity Analysis (Time Complexity & Space Complexity).
[x] I've written my explanation well and it is easy to understand for beginners

[x] I've read and followed the Tutorial Template
[x] I've explained my topic well with 2 - 3 LC problems and no external problems are used.
[x] I've provided the full working solutions to the problems used in this tutorial.
[x] I've provided suggested problems at the end with the given format. See here as an example. If the target solution is not available, leave solutionLink blank.
[x] I've given credits / references if I use external resources. (For an image, give credit under it. Otherwise, add a new section called References at the end (after Suggested Problems).)

[x] I've read and followed the Solution Template
[x] I've formatted my code well with K&R Coding style
[x] I've confirmed that comments are put above each line rather than writing on the same line.
[x] I've included a meaningful approach name for my solution. e.g. ## Approach 1: Two Pointers.