Docs Math.pdf Improvement

Hi all,

I have made an attempt to improve the math.pdf manual. The improvement is
in the section of aligning multi-line equations, chapter 18, the section
explaining how to use LyX to align equations multi-column-wised. As we
know, LyX incorporated AMS Math's align environments into its already great
interface. Although the package is powerful, in my opinion the
documentation can use a bit better explanation and demos. So I did that. I
put a lot of time into it. The original AMS Math's manual is not thorough
on this section neither. I think that's why LyX's part experiences the same
need. I borrowed the concept from the-one-and-only book "The LaTeX Graphics
Companion" where boxes are shown around inputs. And I made many showcases
to demonstrate the package's true power, cuz I think the power is not
properly introduced. The work is not finished yet. For one, I haven’t made
the details to conform with the style convention of LyX manuals, but I want
to present a version so I can get familiar with the flow of contribution.
And also to get feedback and communicate with the docs team.

Drink to LyX’s health and prosperity.

P.S. I attached the whole manual. Not sure if this is the preferred way. So
please go to chapter 18.

Tony