Add a Drupal Global Contribution Weekend landing page

Problem/Motivation

It has been raised the lack of info about Drupal Global Contribution Weekend (DGCW). Currently the only source of details are the single years pages, where the description of the event is 3 lines (see 2019 page), template is not really attractive and most of the information are redundant as being pretty much the same every year.

Proposed resolution

• Create a DGCW landing page in the Documentation space
• Describing what the event is in more/better details (see initial suggestion here)
• Better links to page sections (Resources, Past Events, etc.), either by creating their own sub-pages OR creating a Index section with contextual links (anchors) to better navigate through the page
• Have a template artwork people can share and use (i.e see 2018 page)

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Result

https://groups.drupal.org/node/534740

Remaining tasks

• Discuss if we'll rigorously translate "sprint" to "contribution" in d.o documentation as a followup task, see https://www.drupal.org/core-mentoring/sprint-resources#comment-12943733 →
• Decide if DGCW should have its own "community group" and - if yes - create pages and content (documents in this issue are a good source of content)