Format Exercise 7 to match other exercises

[quan-tob]

Update Exercise 7 to follow the same format as Exercise 6 and 8:

• Fix capitalization in "Table of Contents:"
• Add Context section with link to challenge description
• Add Hints section with links to hints branch
• Add relevant contract list
• Update solution link to solutions branch

Closes #159

🤖 Generated with Claude Code

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Quan Nguyen seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}