Restore markdown guide for about site

Restores the markdown guide for the www-gitlab-com files that populate about.gitlab.com, with an added note on where to find the handbook docs.

Restores it to a different location to avoid adding it to the top-level sidebar, with an alias to redirect.

added HandbookContent label

assigned to @cynthia

changed milestone to %17.1

added 1 commit

• 32150050 - Move to product folder

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requested review from @ipedowitz

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@cynthia can you add a redirect for this as well? Thanks!

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merged

mentioned in commit 10bc83fe