Math cleanup pass

[hollasch]

• Use consistent indentation levels for our math blocks
• Use equation alignment where appropriate

[ymherklotz]

In case this is useful, I just noticed that the alignment of italic variables in math blocks didn't use \textit, which means the spacing looks a bit strange sometimes as they are basically independent letters.

I have made the changes for all variables like that in Book3 (https://github.com/ymherklotz/raytracing.github.io/tree/textit-in-mathblocks), and if this is useful and a wanted changed, I could make the same changes in Book1 and Book2.

These changes are based off of dev-major, so they should be easy to merge and up to date.

[hollasch]

Thank you! In most cases, this has only a very slight effect on kerning, but there are definitely cases where it's significant. Here's a good example:

Before:

After:

It's also proper markup, which would be enough to convince me.

Thanks again for the help -- converting the other two books would be most appreciated, based on the dev-major branch. Also, please add yourself to acknowledgements.html, and add a new v4.0.0 entry in changelog.md.