Reconsider the "Overview" section

[nathandyer]

As part of the documentation overhaul in 2022, the Overview page was expanded into its own section. The original page can be found in this snapshot on Archive.org, as compared to the current version here.

While the "What is SecureDrop" page is much more detailed and provides more information, it doesn't quite fit the same purpose as the original Overview page. Arguably, it gets into too much detail for this kind of an introductory section.

This type of information is useful, and should be part of the up-front, user-facing documentation; but one piece of feedback we've received (or otherwise noticed in calls with new organizations) is that there isn't really a "one-pager" that explains SecureDrop at a high level.

We should consider options and either:
a) Add a high-level overview page back to the documentation
b) Re-structure the home page to provide more of an overview, in addition to the navigational structure that is currently there, or
c) Re-work the "What is SecureDrop" / Overview section to be more high-level/at-a-glance/one-page-summary-ish.