website: Replace website's top banner with a higher-resolution version

The current banner is visibly low resolution. I made the following
graphical changes to the banner:

• upscaled by 2x
• overlayed a high-resolution version of the FreeBSD logo which I exported from the Illustrator source file (website/static/logo/logo-basic.ai)
• removed "FreeBSD" typography in Photoshop and placed a high-resolution version in the same place (from logo-basic.ai as well)
• created a new ® sign
• applied a small patch of gradient to smooth out the line visible in the top left corner of the banner, where the image borders on the background of the rest of the navbar

Sadly, I was not able to do the same for "The Power To Serve",
since the .ai file does not contain it, and the font used for it is
not available anywhere either. I therefore stayed with the original
low-resolution version for that.

I then made the following changes:

• I included the new banner, removed the old one
• I decided to rename it from logo-red.png to banner-red.png, since it's more accurate, and easier to test (replacing an image file without renaming it was often causing preview issues for me, i.e., the old image was often still displayed intend of the new one)
• I changed all instances of the usage to use the new asset and added explicit height/width where necessary

Reviewed by: bcr, 0mp
Differential Revision: https://reviews.freebsd.org/D45782