download page updates post-experiment (part 1)

Hi,

As Dave mentioned in some earlier thread, we've been conducting an
experiment over the outbound links on the downloads page so we can
further optimize layout. We've collected several months of results, and
per analysis of that, we have a few proposed changes.

As this is effectively "release eve" and the lag between "discussion of
experiment" to "implementation of patches" was longer than I wanted, I
wanted to propose the least invasive, best for our user patches that
would require limited discussion.

(I will publish a proposal for the ones that require more discussion later).

As such, the goal of this patchset is to make things even clearer for
our users on what they can download and ensuring that it is a good
experience.

Without further ado, here are five patches:

* Remove the "Build from Source" section on each of the Linux package
pages. The traffic to those pages overwhelmingly (> 99+%) went for some
form of pre-built package; people who wanted the source code would grab
it based on the URL off of the main downloads page (which had a much
higher clickthrough rate).

* URLs to the BigSQL packages are removed as the website is no longer
available.

* Retitle the Linux distro packages to "native packages"; removed Debian
backports section due to lack of traffic.

* Shorten the description on the macOS page and reorder the installers
based on the overall traffic numbers.

* Remove cross-platform Linux installer due to lack of outbound traffic
relative to the content on the pages.

Unless strenuous objection, my plan is to publish this prior to the
release.[1]

Thanks!

Jonathan

[1] Yes, I understand this is short notice, I apologize. But for those
who worry about content changes prior to a major release, there are _a
lot_ more content changes that occur to prepare for the release ;)