slightly refreshed download page

[Daniel Stenberg]

FYI,

After we introduced the official curl builds for Windows[1], I proceeded and
slightly refreshed the look of the download page[2]. Here's what I've changed
so far:

- I introduced a "prio" field which allowed me to sort packages within a
specific OS version, and using that I made the official curl builds appear as
the top-most choice under the windows labels. (I also adjusted the "download
wizard" accordingly so the official windows builds are listed as the top
choice.)

- I made sure "win32" and "win64" are called "Windows 32/64 bit" instead in
the table. Much more relatable name for most users. They're still using the
shorter name in the lockup-thing on the right to remain compatible with
existing bookmarks.

- I modified the name that appears in the leftmost column in the table so
that it rather uses the OS name instead of the package method (zip etc), which
I think makes each line make more sense when looked at stand-alone.

- Removed the term "Generic" from the table. It's just blank now. This makes
32 bit Windows builds for example appear as "Windows 32 bit" and "Windows 32
bit - cygwin". Cleaner.

All these changes are rather subtle and mostly unnoticable.

[1] = https://curl.haxx.se/windows/
[2] = https://curl.haxx.se/download.html