Remove .com subscription info page, redirect to content in docs.gitlab.com

Content was already migrated from
https://about.gitlab.com/gitlab-com/index.html
to
https://docs.gitlab.com/ee/subscriptions/index.html

(per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11026)

This now deletes the file on about.gitlab.com and redirects to the docs page.

Note that https://about.gitlab.com/gitlab-com/ (without the specified index.html) already redirects to https://about.gitlab.com/pricing/#gitlab-com, but this is not reflected in the redirects file. This change should not affect that existing redirect.