feat: adding Sponsor Directory to the menu

Relates to eclipsefdn/it/websites/eclipse.org#827

as stated in this comment I'm doing the following:

1. Rename "Sponsor" to "Become a Sponsor", which will link to the new landing page: eclipsefdn/it/websites/eclipse.org#793
2. Remove "Sponsor a Collaboration", as that page is being replaced by the new landing page
3. Add "Sponsor Directory", linking to this new page https://www.eclipse.org/org/our-sponsors/