Refactor and rename summit to contribute

What this MR does

• Moves the Summit page (/company/culture/summit) to Contribute (/company/culture/contribute)
• Redirects that page to contribute
• Moves a bunch of content to subpages
  • Leadership (/company/culture/summit/leadership)
  • Previous Contributes (Summits) (/company/culture/summit/previous)
• Rewords much of the summit page to use the word "Contribute"

Relevant review app pages

• Old summits page (redirects to Contribute): https://summit-refactor.about-src.gitlab.com/company/culture/summits
• Contribute page: https://summit-refactor.about-src.gitlab.com/company/culture/contribute/
• Leadership page: https://summit-refactor.about-src.gitlab.com/company/culture/contribute/leadership
• Previous Contributes/Summits page: https://summit-refactor.about-src.gitlab.com/company/culture/contribute/previous

Notes

• Length of main page went from 13 pages printed to 8 pages printed (reduced by 40%)
• There is probably a lot of additional design / work that could go into this, but I would say this is better than today and thus we should ship it as an MVC. In the end that's up to @kirstenabma
• Design updated to reflect 'modern' about.gitlab.com pages:

Before	After