Outdated links and minor formatting issues in contributor guide

Some section of the google summer of code contributor guide contain outdated external links and minor formatting inconsistencies . These issues my confuse new contributors who rely on the documentation to understand the GSoC application and contribution process.
1. Open the GSoC contributor guide.
2. Navigate to the contributor documentation section.
3. Click on external links.
4. Observe outdated or broken links.
All links should be updated and formatted properly.
Some links are outdated or lead to invalid pages.
OS : Windows 10
Browser : Google Chrome
Fixing the links formatting will improve readability for new contributors.

Open the GSoC contributor guide and review its contributor documentation section, following each external link to check whether it is current. Update invalid links and correct the noted formatting inconsistencies; done means the links resolve and the guide is consistently formatted for new contributors.

Written by the indexing model from the issue text.