Improve the URL for the top contributors page

Update source/community/contribute/top-annual-contributors/.gitkeep, source/community/contribute/top-annual-contributors/index.html.erb files Deleted source/community/contribute/top_annual_contributors/.gitkeep, source/community/contribute/top_annual_contributors/index.html.erb files

Direct preview link: https://improve-top-contributors-url.about-src.gitlab.com/community/top-annual-contributors/

Closes https://gitlab.com/gitlab-com/marketing/community-relations/contributor-program/general/issues/25