Minor Tweaks based on Issue 63

Based on recent discussion regarding Issue 63:

Fixed open_Projects widget not linking to individual headings
Renamed openings to Careers (careers.md page, URL/Permalink and Navigation menu)
Added Candidate Guidelines to Careers page from https://github.com/vgvassilev/clad/wiki/GSoC-2024
added ajekyll-redirect-from gem and plugin to setup URL redirection for deleted pages (should be supported by Github pages in production, will need to test after deployment)
added to openings page:
```
redirect_from:
- /vacancies/
```

compiler-research / compiler-research.github.io