Add CPU and VERA timing information somewhere

[Yazwh0]

It would be helpful to have this sort of timing information in the docs.

All from @mooinglemur in Discord https://discord.com/channels/547559626024157184/547560914744901644/1473000044956487832

for the X16/VERA?  Pixel clock is 25 MHz.  Therefore it's 25,000,000 / 8,000,000 CPU cycles per pixel.

Total pixel area including blanking intervals in VGA mode is 800 x 525

800 × 525 = 420,000 total pixels.  25,000,000 / 420,000 = 59.523809523809... Hz

and

non-VGA modes are slightly more complicated.  480i has a different number of lines on the even vs odd fields.  With 240p, they are the same.

For all of those modes, 794 is the number of pixels instead of 800 (but I think the clock is divided in two).  So for a full frame (both fields) in 480i, the number of pixels is 794 × 525 = 416,850, which is an average field rate of 59.9736116109... Hz, and for 240p, it's 794 x 526 = 417,644 for a field rate of 59.8595933379... Hz.