An architectural overview of GitLab

This is to help new admins of GitLab understand what components
are involved in making GitLab work.
update links for gitlab.com
Clarifying web frontend direct file access

Rewrite how Apache/nginx interacts with precompiled assets.
updating structure doc
adding ref from structure to architecture
Merge pull request #6185 from sag47/doc/archy_overview

An architectural overview of GitLab